This is just a report of my impressions on recently installing the SOtM.
My set up involves a headless, more or less dedicated, heavily tweaked, low-power PC, outputting via USB to an Audiophilleo2 – a well regarded asynch interface – which runs into the back of an Audiolab M-DAC. The AP2 requires USB power, which is supplied via the AQVOX PSU. These bits were in place for a good while before I installed the SOtM, so any improvement on installing the card are unlikely to be due to reduced jitter (given the AP2) or cleaner USB power (indeed, since the AQVOX is in the chain, the card has always been set not to output USB power). Still, the improvement on installing it was striking – not at all subtle.
The most noticeable improvements were in the bottom end - I'm now getting the kind of bass performance that I knew (from use of my turntable) the system was capable of, and the sound stage has gained both body and depth (before installing the card, it was a little thin and upfront).
Disappointingly (just insofar as this was the only improvement that I was actually expecting) the card didn't help at all with the low-level computer noise, and occasional high pitched ringing, which was coming from the PC and out of my speakers (although I've since been able to remove this entirely).
I can't say I understand how it's doing what it's doing – in my case, it's not obviously a matter of jitter reduction, or improved isolation between the PC and the USB interface – but it's certainly doing something good! I'm now sorely tempted by the battery PSU...