I use an Antelope Zodiac Gold DAC for this very same purpose. I was able to directly compare it to a well-regarded Weiss DAC (FireWire attatched), and I think that the Antelope sounded better. Plus, the Antelope supports almost any I/O option (except FireWire) and supports up to 384 kHz files, both in and out. It's a great product that doesn't receive enough mention, IMO.
Bill
Bill