Trust me you won't notice much of a difference if at all going from the Xonar to the Bifrost. The DAC in the Xonar is one of the best ones. Changing the DAC won't improve your audio as much as the amps and headphones.
If you could get the HE-500 I strongly recommend that. But if not, DT-880 or HD-600, give them a listen first of course.