If only available PCI-e slot is above graphics card that's certainly problem.
In another forum once hit to case of heat from GPU below sound card and CPU above literally causing glitching/malfunctioning of sound card.
Below graphics card would be better position, because heat rises upwards.
What motherboard do you have?
D/A conversion/signal quality wise D2X is certainly still capable.
Most important advances in audio have happened years ago.
For gaming with headphones common drawback of Xonars is use of old Dolby hype Headphone.
Whose bass bloat is just about tolerable with neutral bass headphones like K702.
With above neutral bass headphones like DT990/K712 it sounds rather horrible.
After all we're talking about accurately hearing what's happening in the game and at most controlled bass boost.
And headphone output just has very mediocre 50 ohm output impedance:
https://reference-audio-analyzer.pro/en/report/amp/asus-d2x.php#rw3
Though I guess it should be called positive that it's not 100 ohms, like in most "DX-family" cards and latest gaming hype Strix Raid Pro.
But that would still cause bloating of bass in say HD5xx serie Sennheisers.
Whose lowish impedance rises to 200 or higher at driver's resonant frequency near 100Hz.
And heavy decrease of electric damping factor in most headphones. (related to how tightly signal controls driver)