Well you are going to get a bunch of random responses with few if any consistencies so here's my two bits:
Your primary concern should be a good set of headphones. Open back is the best way to go for the biggest sound stage. I really like the Sennheiser PC363D and PC37x for gaming (yea they are headsets, not headphones)... I'm more of a gamer than an audio guy, so take it for what it's worth. The PC37x can be had for $120 on Massdrop, and it does everything right for games. Low bass and good directional accuracy. What else do you need? I play CS:GO also. If you can hear the footsteps now, there is no "better" setup that hundreds of people on the internet will agree on. I've used dozens of cheaper headsets over the years and the first time I put the Sennheiser 363D on I was grinning ear to ear even with the crappy USB DAC/Amp it comes with. The headset that mutes itself when you push it up is a really nice bonus for me.
Now as for an amp, I bought my first "real" amp recently and I am loving the heck out of it. Does it make all my headphones and headsets sound better? Absolutely! Does it help me perform better in games? Probably not much.
I've been playing CS:GO for probably 10 years now. I'm not great, the highest I got when I was playing alot was LEM before the nerf. You will notice watching pro players that quite a few pros use the Sennheiser GAME One or GAME Zero. They are probably comparable to the PC37x. The key to being a good player is knowing the maps, angles, timings, where to throw nades, where to spam shots when you hear people, being able to flickshot sniper rifles, etc... Chances are spending thousands of dollars on audio gear won't make a enormous difference. Do you have a really good monitor? Like a 120HZ monitor, maybe with GSync? Alot of pros have gone to 120HZ monitors for good reason. Having a good mouse, monitor, and comfortable chair/desk setup is crucial...
Anyway I bought the ifi iDSD black label and I am enjoying the heck out of it so far. Makes these headsets sound great. It is noticeably better than my onboard sound. I've heard people say that it has a good soundstage, and I tend to agree. I will even go as far to say that having the ifi has given me a small improvement to directional accuracy, probably due to the sound separation. But I won't try to convince you to go spend $400 on one because chances are, it won't make you a better gamer.
All things considered the ATH 900X you have may be fine for gaming... I'm really not familiar with it. I do have a ATH900Z on order though and I'll let you know what I think about that if you are interested. If you are unhappy with it give the Sennheiser PC37x a try. I will recommend that over the $380 I spend for the amp I got. They are low impedence so even if you are using a half decent motherboard they should work find for gaming.
I'll stop writing now, but to summarize - make sure you have a decent open-back headphone, and if you are going to spend a bunch of money on a DAC/AMP and headphones do it because you want better sound, not because you want to be a better gamer. I don't know how good you are at games, but I'm willing to bet if you got a decent $100-$200 headset and spent time watching pro videos, set your mouse accuracy right, and practiced alot, you would get better gains than $1000 in audio gear...