I finally got my SRM-T1 back from Accutech after a component went bad, and part of the welcoming party was my new Xonar U3 (to upmix the signal from 2.0 to 7.1 with Dolby Pro Logic IIx, custom remap the spatial processing with 7.1 Virtual Surround, downmix the 7.1 data back to 2.0 with Dolby Headphone, and apply equalization/sound normalization before outputting the resulting processed PCM data through optical TOSLINK) and my Fiio D07K (cheap, effective optical DAC).
In short, my new audio chain is Xonar U3 (processor) -> Fiio D07K + HiFiMeDIY USB Power Isolator (DAC) -> Stax SRM-T1 (amp) -> Stax Lambda Normal Bias (headphone).
The results of this are absolutely mind-blowing. I've never heard anything like this before, even from something like an Astro Mixamp. A Mixamp can't even hold a candle to this, both in processing and in SQ. The spatial processing is so accurate it is almost eerie - I can perfectly place everything going on around me just from watching my uploaded YouTube videos, much less actual in-game audio. When I get my HD7970GEs back from servicing on Monday and start playing again after my month-long hiatus, I wouldn't be surprised if this turns out to be game-breaking.
It works just as well with music, too - I compared the processed and non-processed output while listening to some Daft Punk (the Xonar U3 control panel has a switch to turn all processing on and off) and the result is a vast improvement over vanilla 2.0 - by comparison, the unprocessed signal sounds terrible, or as terrible as a mid-tier Stax setup can sound. My Lambdas sound effortless, but with custom processing, they sound so, so much more so.
So, basically, I've found my audio nirvana. I can't imagine headphone audio ever getting better than this. I highly recommend the Xonar U3 to anyone who listens to music or plays games through a PC, and either has an optical DAC, or doesn't mind shelling out a few bucks for one (the D07K costs like $50, and the Modi is $100). The Xonar U3 costs like $30, and to me, it's worth far more than that.