Best Dac under $600... New or used

mhdt Constantine used would fit your bill to a tee with your budget and probably leave some change if you can find one. It's a SS non-oversampling DAC I use in my main system.

I'd look at the new Parasound Zdac v.2. I have the original model and think that it's ridiculously good for the money. It's very, very close in performance to my Benchmark DAC1 but with a touch of welcome warmth that the Benchmark lacks. The new Zdac has a volume control so it can drive a amplifier directly.

Second Truman above, or Mapman. Can't go wrong.

You could also go for a Micro iDSD from iFi.

I second the Bifrost - with the UBER upgrade - have one - love it!!!