How about 3 options?
1) The Xonar STX (or ST) would give you excellent sound, and it would work well well with the DT990 Pro 250 ohms. With it you would get Dolby Headphone virtual surround for watching movies on your computer.
2) You could get the ODAC/O2 or Schiit Modi/Magni, and the this would be a better DAC/amp setup than the DAC/amp built into the Xonar. The sound would be a little better, and they would better drive a wider range of headphones should you ever decide to get more headphones. But you would not get Dolby Headphone.
3) You could buy the Schiit Modi with optical, the Schiit Magni, and then the Xonar DG (or DGX), and use optical out from the DG to the Modi to be able to use the Dolby Headphone virtual surround. Using the optical connection to the Modi bypasses the DAC in the soundcard, so you get the great sound of the Modi/Magni AND the option to use Dolby Headphone.