It looks like the Xonar has both balanced and unbalanced outputs so it should work as a DAC to feed either Bottlehead amp but to be honest, the review I read of the Xonar was not overly complimentary. I'm not a big HP guy but as a builder and listener to several BH amps I'm sure either the Mainline or the C2A will show how really great they are when fed by the proper DAC and playing through a good set of headphones, matched to show all their strengths. There are numerous threads on the forum (and elsewhere) where others opine their favorite combinations, many with thoughtful details of the sound.
Just my .02, Karl