That Xonar card is no slouch, why not try opamp rolling as your card uses sockets, making switching them easy. Will take a peek at the specs & recommend some!
Asus really know what they're doing, the grounding, layout & isolation of this unit are world-class (digital is much more difficult at gHz than it is in audio at mHz, so their computing background and motherboard design skills help here), could be worth feeding it with a better PSU, is your computer ATX powered?
Mana Acoustics Racks / Bright Star IsoNodes Decoupling >> Allo DigiOne Player >> Pedja Rogic's Audial Model S DAC + Pioneer PL-71 turntable / Vista Audio phono-1 mk II / Denon PCL-5 headshell / Reson Reca >> LFD DLS >> LFD PA2M (SE) >> Royd RR3s.