Originally Posted by DaveBSC
USB is problematic, but so is S/Pdif, particularly S/Pdif driven by a motherboard powered sound card with a single, average quality clock. A high quality USB > S/Pdif asynchronous converter uses two clocks, one for 44.1 and one for 48. Clock synthesis is simply inaccurate, and this avoids that issue, plus you get the clocks away from the inside of the computer, and preferably away from power provided by the computer. Asynchronous mode is not a cure-all (just ask the Hiface), but when well implemented instead of just done as cheaply as possible, it can provide for extremely good results. Indeed, The Absolute Sound's "best digital sound" at RMAF 2011 was a tie between the Empirical Overdrive and Synergistic The Music Cable - both USB DACs connected to Mach2Music servers.
Jitter issues, as I suspected. I don't know if they're that much of a big deal, but I'd have to audition some better equipment first.
However, if the best results are through USB, that presents me with a dilemma. The reason I tend to look at DACs with S/PDIF input and rule out those that only accept USB is that I can still leave a sound card in the chain for its DSP features. From a gaming standpoint, not having the DSP is bad, because then I lose binaural sound, hardware acceleration, EAX, and all that, and with those last two, the sound that the game developers intended (because the software audio fallback omits effects entirely).
I guess the only way around it at this point is to have two separate sound systems for gaming and music if I'm seeking the absolute best performance for each, nevermind my thin wallet.