I have Asus Xonar DX and Fiio E10. They don't work together. PFU picture:
As my head is a tinkering machine I tested my Audio Technica M50s with onboard audio, Xonar DX, E10 and Asus Xonar U7 borrowed from a friend.
This is what I observed:
M50s on onboard audio sounds okayish. Honestly you won't like it. I did not.
M50s on Xonar DX sound much better than onboard audio but they were not really loud enough! The DX has no headphone amp for the front panel. I tested it on the front audio port. Maybe things will be different with sound card with dedicated headphone amp.
M50s on Xonar U7 sound better than the DX (obviously onboard as well) Sound is much fuller and richer I would say.
M50s sound real good on Fiio E10. I can watch movies and listen to music on M50 and E10 for hours. The E10 makes the M50 sound like a discotheque. Although the E10 sounds a bit hollow/not "full" unlike the Xonar U7 I enjoy everything on E10 and would suggest it over the Xonar U7 and Xonar Dx anyday.
PS: I am talking about running the E10 at notch 4/5 on it only. Don't feel the need to crank it up more. It goes up to 8!
If you ask me don't invest in a sound card at all! Get a USB DAC+AMP like the E10 or something even better (I don't know if there are better models at that price range) Since you will be using it with your desktop I suggest to get the E10 as it has no internal battery! Batteries of FiiOs die fast. E10 works on USB power and gives raw power lol.
PPS: Just to add more I tested the M50s with Onkyo HTS3500 as well. M50s sounds better on E10 than the AVR too!
Enjoy!
PPPS: The FiiO E10 works fine on my 10 meter USB3 cable coming all the way from my desktop to my bed side table but the Xonar u7 didnot work on the same cable lol. But the comparison was done on small stock cables.