Best used DAC under $750.

IF needing USB you will want to use a USB/SPDIF converter with either the aforementioned PS Audio DL III or my suggestion, the Bryston BDA-1.  The going used rate for the PS Audio is $300-350 or for the Bryston BDA-1 is $600-800. I have owned both, the Bryston is somewhat better and has far more inputs. Both have XLR and RCA outputs.  

Schiit Bifrost 2 is very good for the money.  have had mine for a month now and like it very much.

I'll be listing my Eastern Electric MiniMax DAC Supreme shortly for $595.00
I believe that it retailed for $1300. new. It has both tube and SS output.
I don't know much about the others that have been suggested here although I can safely say that the EE blew away the DAC in my Sony SCD-C555ES CD player. It has XLR out as well. Check it out.

Used: Schiit Gungnir Multibit Analog 2

New: Schiit Bifrost 2 (as recommended above).

If I was in the market, I’d get the Bifrost 2 and be done with it.