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

Given your budget criteria I would listen to a Bryston BDA-1. Class A analog outputs make this very musical and smooth sounding. It has USB inputs as well as coax, AES/EBU, Toslink.

another vote for PSA's PWDII. it's the best sounding dac i've ever heard at any price (although i've never auditioned a dac above 10k USD so grain of salt might be required here).

this months review in absolute sound also gave it a thumbs up, even when compared to units costing 2x and 3x as much. it's great for redbook but where it really shines is hi res imho.

forget about usb. i'm not a fan and am yet to hear a usb hook-up that comes close to using the PSA bridge.

best bet is buying a used PWDI and upgrading it to II status and getting the bridge. you're still well south of your budget limits but sacrificing nothing regarding sound.

Easily the PWDII. As the review in the current TAS states, it is "one of the most musical front ends at any price." I couldn't agree more.

HEGEL HD11 best i ever try.

It is interesting to hear all the positive comments on the PWD but then I have read the Zodiac Gold + Voltikus when combined is slightly better and this is what is driving me crazy. I am always concerned when spending this kind of money that there is a new flavour every month. Is that the problem with dacs; are they just strictly preference and money really doesn't buy you a better dac?