DUDE. Ok, I've owned the 2010 A40 system and the 2012 system. I sold my headset recently to use a headphone instead of them and I can HIGHLY recommend the AKG Q701 + AntLion Modmic 3.0 combo. The sound stage is so precise with this setup. I use my MixAmp Pro with them and they're SO much better than the A40s ever were.
I will NEVER buy a headset again, they're over priced. You can get a MUCH better headphone for the money and use a ModMic to make it a headset. I have owned the Senn HD555 (basically the same as the 558, that's just the newer edition) and it was not good for gaming IMO. The MMX (DT770 with a mic) is going to be horrible for staging. You need an open headphone for gaming to get a better sound stage.
Just get the Q701 and call it a day man. They're amazing for gaming. Another couple contenders to factor into this that you would also have to buy a modmic for is the Phillips Fidelio X1 and DT990. They have a little more bass but the sound stage is very good for gaming there as well.
The splitter is 3.5mm audio\_______TRRS to plug into the MixAmp
3.5mm mic /
Works perfectly, people online say it sounds "crisp and clear". I play on Xbox.
I would definitely recommend open air headphones over the A40s. I have Audio Techinica AD700s, but they are discontinued and now over priced. The new version are apparently built cheaply.
I would look around. You will come across a nice set, for probably cheaper than the A40s. The A40s are trash. I can't even wear them. You can buy mine if you really want another pair.
Open air though. That's what you want and what will give you more "depth detail" in judging where enemy foot steps are and how far away. Also easier to seperate noises like gun fire from foot steps with a larger "sound stage."