What is the best sounding DAC at $2000-$4000?

I have never heard one but the new PS audio perfect wave II from what I've read sounds like it would definitely be worth checking out. Beyond that my best sound has come from a bryston BDA 1. The benchmark DAC for much cheaper isn't bad at all either and is a super deal at around 600 used.

The Ayre QB-9 is an excellent choice and fits within your budget.