I'm runnign my DAC directly to my amp, and it sounds much better because my preamp sucks.
I'm using ASIO4ALL so the windows mixer is off (there are no dings, beeps, etc) and I lower the volume with foobar's volume control. It sounds really pretty damn good to me.
HOWEVER. When I close foobar, ASIO4ALL becomes inactive, and the noises go straight through to the amp. If you're not careful, you can send dangerously loud sounds to the speakers. It's also quite noisy this way because any hiss/hum from your system is essentially amped at full volume, while you're volume control is reducing the the music only.
I just ordered a pair of these scott endler attenuators
for $80. I'll attach them to the back of the amp, knock them down a few clicks, and hopefully be good to go.