Benchmark DAC-1 USB. That is what I use with my Mac Mini and IMHO, I think it is the best in its class. It has a very powerful output stage with volume control. If you don't have analog source, it can be used as a preamp as well.
American made USB DAC's
I am going to build a music server using a Mac Mini and I'd like to know about some of the American made USB DAC's out there. I'd like to stay around $1000 or so, new or used. Any thoughts? The rest of my system consists of a pair of Magnepan 3.6R's, Modwright KWA150 amp and Modwright LS 36.5 preamp.