What is the best used sub-$500 DAC?

Museatex Bitstream.

Concur with the Museatex, but will also add the EAD DSP7000 Mk.III.

I'd pick up a used MHDT Paradisea or a buy pre-built Monica 2. Both are very organic.

I have a DAC-ah - around $220 new - 8 parallel chips - NOS (non oversampling - filterless). Plenty of good reviews and fans of it here. But then I have not heard a lot of DAC's except I can say it sounds a lot better richer, deeper soundstage than my source unit's built in processors.

Second the MHDT recommendation. I use the Renaissance II. Before that had a Heart CD tubed cdp and the Lite Audio DAC60.