Friend of the Shack
Send the optical output from your PC to your preamp. I have been doing this for years with excellent luck, and sound quality is indistinguishable from the original CD (when sent from CD player to preamp. Of course, it should be indistinguishable from the original when sent digitally, so no surprise there. Be sure to go through all the settings on the sound card's drivers/control software. It seems like there are always a bunch of cheesy surround modes, and it's possible that one might be on for some weird reason -- set everything to "pass through S/PDIF" or whatever it's called on your system, and you should be good.