If you want the best positional audio, I suggest getting a Sound Blaster Z or Omni and using virtual surround. Upgrading your DAC/Amp or headphones will do nothing to improve the positional accuracy of stereo gaming, AFAIK. I mean, there are ones that are rated slightly better than the Senns, but the difference won't be worth the money spent IMO.
This thread is all about people who don't believe it's a gimmick. Chicolom posted a video a few pages back that compared stereo, DH and SBX on the same exact clip from a game. Why don't you go listen for yourself and see if you can tell the difference and whether you think it's a gimmick?
http://new.head-fi.org/t/534479/mad-lust-envys-headphone-gaming-guide-update-7-9-2014-ultrasone-hfi-15g-added/25095#post_10770608
I should rephrase what I said about virtual surround. Some games have good coding regarding the position of the sound (like counter-strike) and don't require virtual surround. Other games (like BF, COD and others), don't have good audio coding and REQUIRES virtual surround to hear surround.
As you can see in this video (disable virtual surround and close your eyes): https://www.youtube.com/watch?v=IUDTlvagjJA it's clear that with good coding, you don't need virtual surround.
As for the footsteps in CS:GO, I believe it's mid-range audio and that's why I'm asking if a soundcard or DAC/AMP would improve on this and if yes, which one is the best to do it. My guess is that both are good and if I only have a 50 ohms pair of headphones, a soundcard will be cheaper and do the same thing but if I'd have a 250 or even 600ohms pair of headphones, a DAC and AMP would be my best bet?