Here's a good thread about what are the best USB dacs.
The reviewer A/Bed alot of great USB dacs from the
Acoustic Arts Tube-DAC / Audio Research DAC8 / Weiss DAC202 / dCS Debussy / Meitner MA-1 / AMR DP-777
http://www.computeraudiophile.com/f6-dac-digital-analog-conversion/dac-reviews-acoustic-arts-tube-dac-audio-research-dac8-weiss-dac202-dcs-debussy-meitner-ma-1-amr-dp-777-a-9680/