I've been using an Asus Xonar DG (rather cheap compared to yours) with Beyerdynamic DT 770 Pro for years and don't have any problems with directional audio except the few bugs ingame. Personally, for headphone use i do not recommend using Dolby Headphone or additional "surround sound" settings - as it is with Dolby Headphones it sounds quite artificial, like tinny and distanced, in my humble opinion, though this is a matter of taste of course. And your brain is quite capable of calculating direction and distance with just two ears without effects ("just stereo", amazing!). Any other additional sound effects just change the original experience and probably won't help with directional sound (depends on tech though).
As far as i can tell, the recommendation is to use 6 channels (5.1 setting in windows) for headphones without any additional effects like dolby or surround, but that info is from a few years back, not sure if still ok (works, though). 8 channels (7.1) requires an audio source that actually delivers 8 channels for proper utilization.
One last thing to note is that it is not possible to ALWAYS pinpoint enemies. The further away the sound the more diffuse the direction, or you might hear one enemy but get surprised by another sneaking around (no footsteps), and it's also possible that terrain and surroundings influence the perceived direction, which makes sense. However, in an open field you should be able to pinpoint a shooter without problems, test this with a buddy to be sure it works.