What DAC are you using? Many USB enabled DACs aren't doing it right and will give you less than the best results.
Best Digital Input from PC to DAC
I'm currently using a Kimber USB cable from my PC to my DAC to play FLAC files ripped from my cd collection to a hard drive. Although the results are quite good I'm interested in other Goners experience with USB or alternate input choices using a lap top computer for manipulating music streams. I've found that FUBAR and the free ASIO drivers render the best results in play back.