Do all of those DSP features work over S/Pdif, or for that matter with a stereo DAC that only accepts stereo PCM signals? I think the best option is to use a dedicated player for computer audio anyway. Something like the Auraliti PK90 USB is very affordable. You can connect it to a DAC that has both asynch and S/Pdif inputs (DacMagic Plus, etc), and just switch the DAC back and forth between sources as necessary.
Yes, at least on X-Fi cards. I confirmed it myself with an SU-DH1, running in straight DAC mode. No Dolby Digital Live or DTS Connect on the computer side, no Dolby Headphone or Pro Logic II on the SU-DH1 side. Hardly an audiophile DAC (analog output's hissy and worse than the X-Fi Titanium HD itself, though I never bought it for PC use), but that combined with RightMark 3DSound's positional test tells me all I need to know regarding the DSP effects over S/PDIF.
The PK90 USB seemed really expensive at first, being $600 $750 (wait, they INCREASED the price?), but then I realized that it's not a DAC, but a Linux-based silent HTPC with some USB ports given their own power supply and all. Might be neat to have if I could put it in a room upstairs, network it and draw the music files from my desktop downstairs, free of all the fan and hard drive noise...but I'd want a significant discount first. (My wallet's even thinner than most here.)