Don't know if this is the info you're looking for, but here's my take/experience with a "pc based preamp for audio":
I use an HTPC(Home Theatre PC) with my stereo(no surround), which in my case is really just an Asus-/Intel-/Vista-based computer with a fairly large(1 TB) Samsung harddrive and an RME HDSP 9632 soundcard, all put into a nicely looking Silverstone cabinet. All my music is stored lossless on the harddrive and played via Windows Media Player - which is said to house an excellent digital volume control, and I simply use the WMP as a preamp, with the analogue output from the RME soundcard fired directly into my poweramp. Using the WMP volume control, as well as the analogue output from the RME, won't let you run in the so-called exclusive mode(which bypasses everything between the raw stored music-data and the outputs, at least as I understand it), but still, believe me, the sound is wonderful - which is likely in no small part due to the excellent WMP volume control as well as the fine work having been done by the skilled people on the Vista sound!
Many will make you think that the internal environment of a computer is a hostile place for an internal DAC(like the RME), and this may theoretically, to a certain extend and from a certain point of view, be true. However, in practicality I wouldn't let this hold you away from trying a high quality internal soundcard(like the RME or Lynx) as your main signal source in a high-end stereo sound system, simply because the sound is indeed great. Using an external DAC, over the solution of beforementioned internal DAC's, may be the ultimate way to go, but you'll have to pay the price for a significant step upwards i SQ.
I'm pointing out the above because it's my impression many are "scared" away from using the pc-based solution - both as storage for your music, playback(especially via internal HQ DAC), and preamp(have only tried out WMP) - and it's a shame since the result is really worthwhile. Perhaps it's the hifi "milieu," and its general lack of acceptance for anything related to PC's as a serious solution. Snobbery also abounds...
I can only say: try it out, and trust your ears..
[PRECAUTION: make sure your poweramp is turned off when switching on and off your HTPC, if used as recommended above]