sennheiser momentum on ear. i own a pair of these and i say the sound is warm, good bass and mid. high is clear too. generally it is a quite balanced headphones with mid and bass slightly forward-ish. but the clamping force is... i could only wear those for an hour max before i have to let my ear to rest for a while. but i guess it's normal for on ear headphones unless you get a pair of bose oe2 (beware of fakes in plaza lowyat though).
i think sound leaking is not an issue anymore because almost everyone in the lrt has headphones/earphones on (thanks to iphone now everyone is listening to music, but with apple's cheap earphones and they leak, A LOT! )
i am using a pair of akg q701 and i say it is perfect for everything. i am on Asus G53sw so i dont have a pcie sound card but im using asus xonar u3 usb sound card though. so i just tune the eq to treble when playing cs:go for sound whoring and default when doing everything else.

comfort-wise, it is comfortable for me but yeah, the headband is a little bit of getting used to but i am used to it now.

sorry it is a opened-back headphones, but sound quality is top-notch.

for amp, my setup = xonar u3 -> fiio e12 on +16db gain -> akg q701, no problem whatsoever.

Yes. I play counter strike global offensive with it everyday. Footsteps are as clear as day and sound positioning is good as well. But of course, you need to have a soundcard for that and not playing it with stereo. But spending over 1k for a headphone to play games is a waste. Buy a ad700 if you need something for gaming only. I use q701 for everything.

Not even close. The AD700 has a larger soundstage and details are mostly easy to pick up, but the Q701 is a lot more refined and a much better headphone for all-rounded use.
The AD700 really only does justice to competitive gaming and nothing else (unless you absolutely love almost-zero bass) IMO.

By competitive gaming, I literally mean competitive gaming — FPS only.
If you're going to use the AD700 to use it as a fun gaming headphone, you'll be dissapointed.

Xonar DGX, but make sure you use UNi Xonar drivers instead of the official Asus drivers.

Using CMSS-3D? Yeah, in theory.
But there's two lingering issue with the THD, output impedance and price.

The THD's output impedance is at a stupidly high 33ohm and that by itself... is even higher than the nominal impedance on the AD700. Not a good match, eventhough it isn't a major issue.
Price though, is something that is a bigger problem. I know there are some shops still selling the THD, but they're going for about RM600 which I don't think is a good deal considering you can order one off Amazon for half of that.

I honestly think it's a matter of preference. CMSS-3D in theory is better than Dolby Headphones (and in the same case, better than THX TruStudio and SBX Surround as well), I say in theory because over the years of using CMSS-3D, I've come to realized I like Dolby Headphones better yet I don't actually lose a whole lot in positional accuracy using it.

With the advent of HRTF coming back into games (AMD's just announced TrueAudio), if it takes off then I don't see the need in virtual surround technolgies anymore on PC at least.
There's also DTS Heapdhone X which at least on paper, is the best blend of CMSS-3D and Dolby Headphones. But as to when that will even see the day of light, that's another question.

If you've got not budget issues, I guess it's fine to go with the THD (mind you, you'll have to use modded drivers) and compensate the impedance mismatch with a cheap, clean amp.