DAC for 300-400 $

For just slightly above your price range, a California Audio Labs Alpha would be excellent.

Michael

I think you should try a Grant Fidelity Tube DAC-11 or an older DAC-09 with a Western Electric tube in place of the stock one. It can power the Sennheiser HD-650 with no problem and is a great sounding combination IMO.

ok, I'll bite. I shopped heavily at the same price point.
Schiit, Arcam rDac, Musical Fidelity VDAC II were at the top of the list. I ended up with this and am quite happy.

Valab WM8741 24bit 192k USB DAC --- Dual WM8741 (mono mono configuration) Balanced output
It is now $380 delivered. The Valab has the well known Tenor USB interface, which does 24/96, but is not Asynchronous. I ended up also buying a Lars Audio USB interface that is Asynchronous, uses galvanic isolation on the power supply and processes with a similar chip of the Schiit Bifrost, does 16/44 through 24/192 (thats all i've tried) Excellent build. This has been a very satisfying combination. Rich, textured, detailed, tight and fast.
Good Luck, there is alot of nice gear out there, find the one right for you. Tim

Have you considered buying an amp/dac combo? the HA-160DS is pretty good headamp and it comes with Dac. Just a suggestion.