I have been running PC audio directly to my amp for a while now and have been thrilled with the sound. My equipment:
HTPC with Asus Xonar D2X
Bryston 4B SST for the fronts
Emotiva UPA-2 for the rears
MartinLogan Prodigy fronts
MartinLogan Aeon i rears
MartinLogan Descent subwoofer
No receiver, no center.
The sound is a huge step up from the Pioneer Elite receiver I was using previously as a preamp/processor. The detail from the D2X is just amazing, and really not surprising if you compare the specs of the card with a typical receiver, especially when it comes to signal-to-noise ratio (118dB).
If you think about how little a high-end sound card does compared to a typical receiver, it's not surprising it performs so much better in the one crucial area (sound). There's just not much margin for using expensive components in a receiver. The D2X uses Burr-Brown PCM1796 DACs, the best in the business.
1. To the guy saying connect the mobo's analogue outs directly - horrible, horrible advice.
2. The Asus cards handle bass management.
3. I love the D2X, but the Essense ST + H6 is supposed to be even better (124dB). One of the main downsides of the D2X is that the analogue outputs are 3.5mm stereo jacks instead of mono RCA outs.
4. My previous Pioneer receiver had Audyssey room EQ but I don't miss it.
5. You CAN connect an external audio source (with one big limitation). I have my PS3's SPDIF output connected to the sound card's line-in, it's digital, but only 2-channel (AC3/DTS doesn't work).