Flagship Electrostatic Headphones Review: Audeze CRBN & Stax SR-X9000

Flagship Electrostatic Headphones Review:
Audeze CRBN
Stax SR-X9000

Rajiv Arora

After my highly enjoyable time reviewing the latest flagship planar magnetic headphones, I next turned my attention to electrostatic headphones. Partly, this was prompted by the recent releases of the Audeze CRBN and Stax SR-X9000 flagships, but I have been interested in exploring this class of headphones in my own system for quite some time.

Over the years, I have dutifully listened to various electrostatics at CanJams and other audio shows. While I could hear the alluring qualities of these headphones, it always seemed to come at the cost of excitement, dynamics, and bass. Could this simply be an artifact of show conditions and/or limited upstream components? Would I have a different, and more magical experience in my own system, one that I’ve spent years improving and fine tuning? This is what I wanted to find out.

My Listening Setups

Baseline Reference System

I recently wrote an article describing the latest round of changes in my reference system and the rationale. My baseline reference system for this review is shown in the picture above. It consists of a chain of audiophile switches, feeding my music server, the Taiko SGM Extreme, equipped with the Taiko USB card upgrade. Audio data is output over USB to a Vinnie Rossi L2 DAC module in an L2i SE integrated amplifier. My reference headphones are the Meze Empyrean, the Abyss AB-1266 Phi CC, and a modded Sennheiser HD-800 (SD mod). The switches are powered by independent DC rails supplied by a Paul Hynes SR7MR3DRXLFC10 power supply. AC power is delivered via a 6AWG dedicated circuit to a Sound Application TT-7 Reference power conditioner, to which the amps, the Extreme, and the Paul Hynes PSU are directly connected. Additional details are available in the System Details section below.

To drive my reference headphones from the L2i SE speaker amp, I used the functionally versatile and excellent sounding Transparent Ultra Headphone Cable System, which comprises an Ultra banana plugs-to-female XLR adapter, the Ultra cable, and then headphone-specific leads for each of my headphones. This enables me to switch between headphones whilst using the same cable.

Since this was my first deep dive into electrostatic headphones, I first wanted to establish a reference baseline of electrostatic headphones for myself and my ears. What better headphones to do that with than the venerable Stax SR-009S! Stax Japan and Edifier were kind enough to loan me both an SR-009S unit along with a Stax SRM-T8000 electrostatic headphone amp ahead of the arrival of the SR-X9000 review unit, to enable me to acclimate to the electrostatic sound.

Review System Topology

When it came time for the headphones under review, I also had on hand, thanks to a generous loan from an Audiophile Style forum member, the highly-regarded Mjölnir-Audio Carbon CC electrostatic headphone amp. This also allowed me the luxury of comparing electrostatic amps. With all this cool hardware on hand, it was time for some listening!

Review Playlist

Flagship Electrostatic Headphones Review Playlist

To enable you to listen to the same tracks that I did, I have created a public playlist on Qobuz USA. This playlist includes the tracks mentioned in this review, as well as some of the others I listened to in the course of this evaluation. Please note that in some cases, the Qobuz track will not match the mastering I listened to, especially since all my listening was with PGGB-upsampled files, stored locally on my Extreme server. Still, this gives you a sense for the music I listened to for evaluation.

Baseline Listening: SR-009S with the T-8000 Amp

I used Stax’s SRM-T8000 electrostatic headphone amp (US MSRP $6090 white, $6825 black) to drive the SR-009S headphones (US MSRP $4545). The T-8000 is a hybrid tube/SS amp. Per Stax’s website, “It employs two 6922 in the input stage, driving a Class-A solid-state output stage.” I used the volume control bypass, which allowed me to use my Vinnie Rossi L2i SE as an external preamp.

I must say, hearing the SR-009S in this baseline reference system was a pleasant shock. This was not the polite electrostatic sound I had heard in my various auditions of these headphones at audio shows! 
 

Listening to the song Ay Hairathe on the album Guru (Original Movie Soundtrack) (16/44.1, Sony BMG Entertainment), music by A.R. Rahman, I was immediately immersed in the lush soundstage created by Rahman’s instrumentation and the intimate vocals. Rahman’s genius comes from restraint, as he interweaves and highlights different instruments and instrument groupings. The clarity and detail with the SR-009S was quite mesmerizing. The tabla bass lines were satisfyingly deep and impactful. Female vocals, as in Alka Yagnik’s voice, did come across a bit intensely.

Alexander Scriabin, Symphony No. 3 &4, Vasily Petrenko, Oslo Philharmonic (24/48, Lawo Classics). I discovered Scriabin’s 3rd symphony on a Portuguese classical radio station, driving down the backroads of the Alentejo one vacation. Scriabin’s kaleidoscopic dreamscape seemed to perfectly capture the mood of the empty roads and the soft winter afternoon sunlight, and ever since, this piece has become an enduring favorite of mine. On the introductory fanfare, the SR-009S delivered the requisite growl of the tubas with convincing weight and body. The piece then flows into the Luttes (Struggles) movement, with phrases that ebb and flow. Here, the SR-009S was captivating in the way it rendered the textures of the harp, in particular, but more generally the way it kept up with the ever-changing melody. This is where I appreciated that famous speed and transient response of electrostatic drivers. The resolution and micro-details of individual instruments as well as massed strings and brass was at a level I had not heard with dynamic and planar magnetic drivers.  

I spent several weeks enjoying this baseline configuration. As I tried a variety of music on the SR-009S, it continued to exhibit its speed and resolution across genres, while supplying unexpectedly good levels of bass and dynamics. Tonally, the palette is somewhat bright, which can be fatiguing on some recordings.

Swapping amps: Mjölnir-Audio Carbon CC

Some weeks after I’d lived with (and enjoyed) the SR-009S driven by the SRM-T8000, a good friend dropped off his treasured Carbon CC amp for a loan spell while he was traveling. 

Mjölnir-Audio sells a line of electrostatic amps built by Icelandic DIY builder Birgir Gudjonsson, based on Dr. Kevin Gilmore’s designs. The Carbon CC (MSRP USD 6900+, depending on customizations) is said to be his current flagship, using the highest grade parts. From the website: “...Higher spec power supply, 99.99% pure silver signal wiring, upgraded TKD volume pot, higher grade resistors for the amplifier channels and finally, new custom made PEEK output sockets…”

Given the limited time I had, I got to work immediately deploying the Carbon CC in my system. Since this amp doesn’t have a bypass mode, I fixed the volume control at 12 o’clock and did all the volume adjustment on my Vinnie Rossi’s preamp. This was a well-burned-in unit, but I gave it a day to settle in my system before any comparisons.

Which did not take long.

As good as the SRM-T8000 sounded, the Carbon CC took things to a different level. The bass got deeper, tighter, and more impactful. Instruments took on a solidity that made them sound all the more real. There was an increase in dynamics as well. The Carbon CC also seemed to dial down the SR-009S’s inherent brightness, which was very welcome. Finally, there was an added bit of richness and refinement in the Carbon CC.

Based on these results, I used the Carbon CC in all the subsequent evaluations and comparisons. The system is shown in the second diagram above, labeled “E-stat Review.”

Stax SR-X9000

The SR-X9000 (US MSRP $6200) are Stax’s new flagship electrostatic earspeakers. For details on the technology advancements in this new flagship, I refer the reader to https://staxheadphones.com/collections/earspeakers/products/sr-x9000.

Visually, the SR-X9000 is a stunning pair of headphones. The finish of the leather headband, the aluminum sound units, the stainless steel arc unit, and the leather earpads are all at a level of elegance befitting a flagship.

As always, my primary concern was comfort, and here the SR-X9000 was a champion. These are some of the most comfortable headphones I’ve worn, and almost as good as my current benchmark for comfort, the Meze Empyrean. The round earpads fit over my large ears without problems, and the sheepskin surface of the earpads was so soft that I hardly felt them at all. Clamping pressure was just right, even on my large head. The SR-X9000 weighs in at 432g, even lighter than the SR-009S.

The review unit came with a 2.5m detachable cable. This cable is said to comprise an inner core of  6N OFC pure copper, surrounded by silver plated annealed copper wire. This cable was soft and flexible, and had a flat profile that made it very comfortable in use.

Listening Impressions

My review unit had been around the block a few times already, so I found no burn in was necessary. In fact, from the first track I listened to, the SR-X9000 had me hooked.
  

There is nothing particularly audiophile about Suzanne Vega’s 1987 album Solitude Standing (24/96, A&M), but I was in the mood for some 80s indie folk rock. Listening to Ironbound/Fancy Poultry, the SR-X9000 managed to dig deeper into this decent-enough recording than any other headphones have. Compared to the SR-009S, the SR-X9000 sounded smoother, richer, and had better micro-details, while the 009S seemed a tad more dynamic and brighter. Bass sounded fatter and deeper on the 009S, but tighter on the X9000.

Brahms Piano Concerto No. 2, Haydn Variations, Lars Vogt, Royal Northern Sinfonia (24/48, Ondine). On the final Allegretto grazioso movement of the concerto, the immense soundstage projected by the SR-X9000, coupled with its speed and resolution rendered the music with intense realism. The clarity and crispness of the piano strokes highlighted the virtuosity and speed of Vogt’s playing. Crescendos, when they arrived, were satisfyingly prodigious, although I found the SR-009S to still have an edge in dynamics. In all other aspects, the SR-X9000 took the lead. Even on bass, the SR-X9000 went just as deep, while being tighter and better controlled.

I listened to the SR-X9000 extensively for several weeks, both genres I usually listen to, and some I hadn’t in years. It handled everything with aplomb, and my biggest problem on most nights was ending my listening sessions. 

The differences I noted between the SR-X9000 and the SR-009S in the examples above proved to hold even with extended listening. 

1. Speed and transient response were to be expected in an electrostatic headphone, but the SR-X9000 extracted more detail and texture from instruments, which also manifested as a richness in tone that was quite beguiling. 
2. Bass was tighter and more controlled than the SR-009S. Even though subjectively the bass did sound deeper on the latter on some tracks, I preferred the control and articulation of the SR-X-9000. 
3. Another area where the SR-X9000 excelled was in the image or soundstage size, which was larger in all 3 dimensions that the SR-009S.
4. Finally, I found the SR-X9000’s tonality to be neutral and natural, with instruments sounding more real. The SR-009S appears a bit bright for my tastes.
5. The one area where I found the SR-009S to retain superiority was in dynamics, sounding snappier than the X9000, which could sound ethereal on some tracks.

If it isn’t already clear, I loved these headphones, and considered them a significant step up from the SR-009S.

Audeze CRBN

Arriving right on the heels of Audeze’s planar magnetic flagship, the LCD-5, comes their electrostatic flagship headphones, the CRBN. As their first and only electrostatic offering, the flagship moniker could be redundant, but make no mistake, this is a flagship – in price (US MSRP $4500) and in intent. Can this new entrant from Audeze compete with the established Stax-ocracy of the SR-009S and the SR-X9000? I can’t wait to find out!

The design story of the CRBN is both inspiring and intriguing. Audeze developed a novel thin-film diaphragm “with suspended carbon nanotubes” as part of a research collaboration with UCLA “to create a patient-friendly headset for use in MRI machines.” Having endured my share of noisy MRI scans, hats off to Audeze for this work!

This diaphragm also proved to be the basis for the audiophile CRBN headphones. Audeze found this design had several advantages, particularly in the area of bass extension. As explained in the link above:

Electrostatic headphones have a reputation for less than stellar bass response. The Audeze solution to this well-known issue involves several angles. Our carbon nanotube diaphragm allows us to achieve high efficiency by holding more charge and preventing charge migration. This allows us to reduce diaphragm tension and increase the space between the stator and diaphragm, which provides for higher excursion and lower resonant frequency without the risk of the diaphragm coming into contact with the stator. This low resonant frequency, combined with a large diaphragm surface area, acoustically transparent damping material, and custom designed earpads provide extremely well-extended bass response down to 20Hz.

Fit and Function

It’s easy to tell the CRBN and the LCD-5 are siblings from the same manufacturer. They share a similar design aesthetic. While not quite as light as the LCD-5, the CRBN still weighs in at a svelte 470g, which is comparable to the Stax units under review here.

Attractive as the LCD-5 is, the CRBN takes looks and aesthetics to a new level. Starting with the elegant etched grill, the polished rings, the leather headband, all the way to the soft leather earpads, these headphones exude class in a way the LCD-5 does not. But then, the LCD-5 is not the competition; the Stax 009S and X9000 are! And as a piece of audiophile art, the CRBN  holds its own very well.

After my challenges with getting a comfortable fit with the LCD-5, the CRBN was a pleasant surprise. I had no problems finding a comfortable yoke setting, and clamping pressure was not an issue at all on my largish head. The large, plush ear pads contained my ears without any contact with the grill. This is something I run into a lot, so high marks to the CRBN on this score. While I still wish the yoke could be set on a continuum, rather than at granular detents, this did not prove to be an issue in practice. Comparing comfort head-to-head, I would still give the nod (ok, enough head puns!) to the SR-X9000, but that does not change the fact that the CRBN are very comfortable headphones.

Listening Impressions

To get a baseline of the CRBN’s sound quality, I compared it to both its planar magnetic sibling, the LCD-5, and its electrostatic competitor, the Stax-009S.
 

Ottmar Liebert’s album Up Close (24/96, Bandcamp) has long been one of my favorite binaural albums that I enjoy primarily for the music. That said, it’s both an outstanding performance and recording. On Carousel, the CRBN immediately had me enthralled with the speed and detail with which it rendered the intricate rhythms of the guitar with the hand claps, far more so than the LCD-5, which already has excellent transient response. There’s just something about electrostatics! The CRBN’s bass response on this track was the equal of the SR-009S, but both didn’t quite match the depth of the LCD-5. Still, the CRBN’s bass was impressive and prodigious, both in the upper bass with the bass guitar, and in the deep bass when the drum solo kicks in at the 5-min mark. Tonally, the CRBN sounded smoother and more neutral than either the LCD-5 or the SR-009S.
  

Gustavo Dudamel hasn’t always convinced me of his Mahlerian chops in the past, but his latest recording of Mahler: Symphony No. 8 (24/96, Deutsche Gramaphon) has earned a place in my favorites. Much is also due to the polished playing of the LA Philharmonic, the acoustics of Disney Hall, and Deutsche Gramophon’s recording. Mahler’s “Symphony of a Thousand'' really benefits from electrostatic headphones to tease apart the myriad harmonic lines and dense orchestration, and the CRBN does this superbly. One test of headphone quality I apply during complex passages is to focus on specific instruments or groupings and note how well I can follow just those instruments, even when they are in the background. The CRBN did this much more convincingly than the LCD-5, and somewhat better than the SR-009S. Of course, this piece is also one of the most challenging in terms of soundstage, dynamics and bass! The CRBN’s soundstage was larger than the LCD-5, but not quite as large as the SR-009S. The LCD-5 was the dynamics champ here, but the CRBN was not far behind alongside the SR-009S.

The CRBN makes a very compelling case for the superiority of electrostatic headphones. It comprehensively bested the LCD-5 on everything except dynamics and deep bass, but even on those, it did not concede much. The CRBN is more natural, tonally smoother, more refined, and has significantly better transients, speed and bass articulation. Of course, one has to acknowledge the contribution of an expensive electrostatic headphone amp of the Carbon CC’s caliber.

Compared to the SR-009S, I found myself preferring the CRBN on most fronts. While both have excellent and clean bass, the CRBN is more articulate and controlled. Tonally, these are very different headphones. While the CRBN is not as aggressively mid-forward as the LCD-5, it still gives a front-row presentation to the listener. The SR-009S has the larger soundstage, but persisted in sounding brighter-than-neutral over extended listening, making it more fatiguing to my ears. 

Comparing the CRBN and the SR-X9000

Now to the most interesting listening tests: how do these flagships (the CRBN and the SR-X9000) compare to each other? 

One of the joys of Qobuz is browsing new albums, and this is how I stumbled upon Bows Up! Portuguese Music for Strings (24/44.1, Naxos) a few months ago. The first movement of Sérgio Azevedo’s Sinfonietta for Strings, Inquieto, has a hypnotic, pulsing rhythm laid down by the bass, over which the strings of the Camerata Atlântica soar. The CRBN captured the physicality of the cellos and bass in a most un-electrostatic-y way, while still portraying the timbre of the violins and violas with the expected clarity and speed. The X-9000 took me deeper into the music. It portrayed a much larger soundstage, while adhering to what I felt was a tonally more  realistic presentation of the instruments, especially the violins. The X-9000 drew out even more micro-details, and this, combined with the tonal realism, came across as a rich, almost creamy presentation that I could listen to all day long.    

I then switched gears to another of my favorite albuns, Buena Vista Social Club (25th Anniversary Edition) (24/96. World Circuit). On El Cuarto de Tula, the X-9000’s larger soundstage, higher resolution, and dead-on tonality was again very evident, although not by as great a degree. The CRBN was no slouch, and actually came out slightly punchier, delivering a highly enjoyable listen I would not have found wanting, had I not had the X-9000 also at hand.

Comparisons are so often cruel, and this is a case in point. The CRBN is priced comparably to the SR-009S, and on this level playing field, it comes out with flying colors. For a company to come out the gate with their first electrostatic offering and unseat the reigning benchmark is quite a feat. Kudos to Audeze for this accomplishment.

But Stax have not stood still, and with the SR-X9000, they have extended the state of the art in sonic performance in ways the CRBN does not quite match. Of course, this comes at a significantly higher price point. At least in this instance, you do get what you pay for!

Comparing the SR-X9000 with the Meze Elite

Having crowned the SR-X9000 the champ of the trio of electrostatics in this review, and with my time with it rapidly coming to a close, it was time to compare it with my current in-house planar magnetic reference, the Meze Elite. But this was more than a headphone comparison, as the amps involved are different. As I covered in my recent system update, the Vinnie Rossi L2i SE speaker amp was a big step up from my previous Cavalli Liquid Gold headphone amp. Clearly both headphones were being driven by excellent amps in their respective category.

On the title track from I Robot, The Alan Parsons Project (24/96, Arista - Legacy), both headphones took this somewhat bright mastering into their stride, delivering a similar tonality in the midrange and treble regions. The Elite’s bass was more prominent, and it was much more dynamic. It also had a denser, heavier presentation. The SR-X9000’s bass, while not subjectively as deep, sounded cleaner and effortless. Instruments across the board were crisper with clearer leading edges. Finally, the SR-X9000 portrayed spatial cues and ambience much better, and brought out more of the tiny details in Parsons’ lush instrumentation than the Elite.  
  

Next up was Jordi Savall’s eagerly-awaited second volume to complete his recording cycle of Beethoven symphonies. Listening to the fourth Allegro vivace  movement of Beethoven’s Symphony No. 8 from Beethoven Révolution - Symphonies 6 à 9, Jordi Savall, Le Concert des Nations (24/88.2, Alia Vox), this excellent performance and recording was alluring on both headphones. Savall’s performance may be historically informed, but it has a raw energy and physicality to it that is simply thrilling. The SR-X9000 conveyed this much more convincingly than the Elite. Yet when the timpani got going in the crescendos, the Elite conveyed this more viscerally. Here again, the Elite provided more density and body, along with better dynamics, while the SR-X9000 portrayed a larger soundstage, and instruments that sounded more real and natural.

While one might assume that the SR-X9000/Carbon CC combo would run away with the lead, my actual findings were more nuanced. The SR-X9000 is the clear winner on soundstage size, speed, transient response, instrument texture and realism. But when it comes to density, body, dynamics, and bass, the Elite was the winner. It ultimately comes down to the attributes one values more.

Concluding thoughts

This was one of the most enjoyable reviews I have done in quite some time, because it allowed me to experience the best in a class of transducers with which I had not not previously spent much time.

Both the Audeze CRBN and the Stax SR-X9000 headphones are superb components. Paired with an electrostatic headphone amp of the quality of the Carbon CC, and fed by my chain of Taiko Extreme and Vinnie Rossi L2 DAC/L2i SE, they really opened my eyes to just what I had been missing with electrostatic headphones.

The CRBN builds on the DNA of Audeze’s headphones. It retains most of the excellent bass and dynamics of its planar magnetic cousins, then adds in the electrostatic magic of speed, resolution, and transparency. The CRBN really shines in genres like rock, jazz, and EDM, which benefit from its strengths in body and density. Throw in the fact that it is the most comfortable Audeze I’ve tried, and you have yourself a winning combination. 

The Stax SR-X9000 is billed as Stax’s new flagship. With a price tag to match, it has very high expectations to meet. Yet, that is exactly what it does. As amazed as I was at how great the SR-009S sounded in my system, the SR-X9000 comprehensively surpassed the SR-009S to an extent that more than justifies the increase in price. On classical symphonic music, the genre I listen to most, the SR-X9000’s was by far the best music reproduction I have ever heard on headphones. It has a vast soundstage, tonality that is spot on, and it reproduces instruments with such realism in texture and tone that it connected me to the music more emotionally than any other component. The only break from perfection is a tendency to sound ethereal, without quite the same dynamics as its sibling, or the best planar magnetics.

Are electrostatic headphones in @austinpop’s future? If the cost of the SR-X9000 and Carbon CC combo were not quite so daunting, and availability not quite so scarce, I would have pulled the trigger already. As things stand, I remain in the “sorely tempted, on the brink” camp. I have a feeling though that sooner or later, this will happen. The siren song of electrostatics is just too alluring to resist!

System Details

Music Computer:          Taiko Audio SGM Extreme Music Server, Taiko USB upgrade
DAC:                               Vinnie Rossi L2 DAC module
Headphone Amp:           Vinnie Rossi L2i SE integrated speaker amp with Transparent Ultra adapter kit
Electrostatic Amp:          Mjölnir-Audio Carbon CC
Reference Headphones:  Meze Elite, Abyss AB-1266 Phi CC, Sennheiser HD800 (SD mod)
Ethernet Switches:          SOtM sNH-10G, Uptone EtherREGEN, Buffalo BS-GS2016 (modded for LPS)
Power supplies:                Paul Hynes SR7MR3DRXL (dual regulation, 3-rail)  for switches
Power Details:                 Dedicated 30A 6AWG AC circuit Sound Application TT-7 Reference Power Conditioner
Power Cables:                   Sablon King (wall to TT-7), Sablon Prince (Extreme), Cardas Clear Beyond (L2i SE, SR-7), Cardas Clear for all other components
USB cable:                         Sablon Evo 2022 USB
Ethernet cables:                Sablon 2020, Supra Cat 8
DC cables:                         Paul Hynes fine silver (SR-7)
Headphone cables:          Transparent Ultra cable system
Accessories:                      Synergistic Research Tranquility Base XL UEF. Galileo MPC, Synergistic Research MiG 2.0 footers, Taiko Audio Daiza Isolation Platforms, High Fidelity Cables Trinity Helix Headphone Module.

Acknowledgments
 
Many thanks to the following companies for supplying cables and accessories to aid in this evaluation: 
* Cardas Audio, for a full loom of Cardas Clear cables.
* Transparent Audio, for the Transparent Ultra headphone cable with a full complement of headphones leads and source terminators.

About the Author

Rajiv Arora — a.k.a. @austinpop — is both a computer geek and a lifelong audiophile. He doesn’t work much, but when he does, it’s as a consultant in the computer industry. Having retired from a corporate career as a researcher, technologist and executive, he now combines his passion for music and audio gear with his computer skills and his love of writing to author reviews and articles about high-end audio.

He  has "a special set of skills" that help him bring technical perspective to the audio hobby. No, they do not involve kicking criminal ass in exotic foreign locales! Starting with his Ph.D. research on computer networks, and extending over his professional career, his area of expertise is the performance and scalability of distributed computing systems. Tuning and optimization are in his blood. He is guided by the scientific method and robust experimental design. That said, he trusts his ears, and how a system or component sounds is always the final determinant in his findings. He does not need every audio effect to be measurable, as long as it is consistently audible.
 
Finally, he believes in integrity, honesty, civility and community, and this is what he strives to bring to every interaction, both as an author and as a forum contributor.