I would probably recommend the akg 702 over the Senns. Better soundstage and imaging for gaming. Nothing wrong with the hd598/9 of course and to most there wouldn't be a huge difference. Got gaming I would say there is little point in getting any higher Senn headphone such as the hd650 (which would be a tad dark anyway)
Indeed good binaural sound requires certain things from headphones not necessarily needed by music.
With its apparently mediocrish soundstage HD650 is way worser than HD59x models (not to mention AKGs) for competitive fps gaming with focus in hearing foot steps and such. Which benefits greatly from bass neutral/light frequency response and big soundtage.
And in "fun" gaming with immersion of explosions and gun sounds equally important there are other cheaper open headphones with strong bass and big soundstage.
Like DT990 Beyers or Philips Fidelios, which do also that competitive part better thanks to big soundstage.
With "neutralizing" bass using equalizer likely allowing increasing their competitive gaming performance.
AKG 701/702 are light/neutral on bass notably decreasing that fun aspect, while being good for that competitive gaming.
Though apparently there has been slight changes during years.
K712 should be more balancing with stronger more immersive bass obviously making footsteps etc little harder to distinguish, just like in DT990 Beyers.
Myself have HD595 Sennheiser which definitely work extremely well for telling those footsteps etc. But with light bass making that "fun" immersion lowish.
DT990 Beyers are excellent for that fun part. But with immersive bass making hearing those footsteps less excellent, though still great with big soundstage.
DT880 semi open design imitating HD330 Superlux has clearly smaller soundstage likely much like that HD650 Sennheiser.
With stronger than DT990 emphasis on bass and treble that gives "front row" agressiveness to say rock music.
But in binaural game sound that smaller soundstage is clear minus.
That Youtube BF4 comparison is very good for comparing headphones for gaming.
With human hearing having bad memory and being even worser as measuring instrument that close/side by side comparison is important.
Only after getting DT990s really found out how bass neutral HD595 is.
(that fashion design of Sennheiser started getting cracks to plastic parts in couple years)
As with the headphones more could an spent on the amp for better performance placebo.
Now it's correct.
With semiconductors and especially opamps it doen't take expensive parts to make amplifier which is transparent for human hearing.
Of course there are certain situations which have to be accounted for getting absolutely neutral technical performance in all situations, but overal that electric engineering part is clear and well studied.
http://www.tomshardware.com/reviews/high-end-pc-audio,3733-19.html
And many other scientific test have confirmed that after reaching certain (technically not so high end specs) level human hearing simply can't tell things apart.
That's the reason why "audiophiles" don't accept blind testing: It would show all those expensive special cables, equipment etc as scam.