Xonar DX should be at the absolute bottom of your list, period. Buggy drivers, extremely low quality components, extremely bad frequency response clipping and gapping. It is complete and utter crap.
If you want a good, reliable sound card for high quality sound your choices are M-Audio Audiophile 192 (strictly 2/2 pro audio and DO NOT use for gaming, amp required as well) or an
HT|OMEGA. I've been using HT|OMEGA for a number of years and not only are the rock solid reliable, the sound quality is flat out unmatchable even before you swap the OpAmps, and the price is lower because you're not paying for marketing budget and hype.
And this is true of all HT|OMEGAs cards. I'm running a Striker 5.1 in one system for near-pro (it's a little weak on the high), Claro II for semi-pro, and I used to listen to the Halo XT (that's the version with the headphone amp) daily.
Honestly, you might be better off going with the
Halo XT since it's got all the bits in it already. I've been poking around at the various USB DACs for several weeks now, and they're
just not very good. Not till you start dropping several hundred dollars or accept a ridiculous amount of latency.