Just had mine upgraded, well worth the $. My review from WBF:
Two years after purchase I finally had the Aqua Formula xHD upgrade performed. In somewhat of a nutshell here's my experience:
With Alex's help I shipped the DAC to Aqua in Italy which was returned upgraded in about 3 weeks which went flawlessly. Also, while having the DAC upgraded to xHD I had the fuse replaced with a Blue Quantum fuse. In terms of break - in, while most of the upgrade goodness occurs after ~120 hours of break - in I found that at about 200 hours it settled down.
A few setup comments: Before the upgrade I found upsampling PCM to sound best, however with the xHD I now find DSD upsampling to be superior. In my experience hearing quite a few DACs I always thought that upsampling to DSD sounded a bit brittle and thin whether upsamping DSD or PCM. On several Esoterics I noticed this and as such I stayed away from DSD upsampling. That's not the case with the xHD - DSD sounds fabulous upsampling to Quad DSD. More below.
The difference between the non xHD upsampled to 384K PCM versus xHD also upsampled to 384K PCM (highest upsampling capability before the upgrade): Added bass weight and tightness, ability to hear further into the music (more detail), further effortlessness, more detailed spacial cues and in a nutshell just plain more real.
The difference between the non xHD upsampled to 384K PCM versus the xHD upsampled to 768K PCM yields additional information without sounding overwhelming, fatiguing or overly analytical.
xHD upsampling to Quad DSD: I know many folks swear by DSD files (and true native DSD recordings) and I'm now a believer. Playing DSD upsampled to Quad DSD is other worldly good. The amount of information is staggering, the sound stage layering in depth and width is another level better and again, the realism is better than I expected my speakers could articulate.
Net - I'm quite satisfied with the upgrade, worth every penny. I'm in no rush for a DAC upgrade and if I did jump, it would be to the TD12 in the distant future. Thanks for the assistance Alex!