Anything more than that level of bass starts to seriously kill details.
Cheap closed garbage just often use strong bass to hide lack of sound quality...
That along with closed design's soundstage challenges/problems makes "gaming" stuff usually from mediocre to very bad for gaming.
Just listen these in stereo mode without effects with those quality headphones:
Beyerdynamic DT990 might have slightly more feel of bass than K712 basing to rating in starting post.
Should find that out in next week with K712 now in its way to post office.
Though if DT990 really has more (fun) feel in bass than K712 would expect K712 to win in details.
Anyway frequency responses are very similar:
http://graphs.headphone.com/index.php?graphID[0]=4163&graphID[1]=2141&graphID[2]=&graphID[3]=&scale=30&graphType=0&buttonSelection=Update+Graph
While having good binaural simulation otherwise Sennheiser GSX is full snake oil overpriced, lackluster, gaming product marketing scam.
Its DAC and headphone output struggle to match cheap ass Audigy Fx/Xonar DGX. (just look for price of them)
And like you noticed doesn't even have connectivity that luxury price should give.
Lot cheaper Sound BlasterX G5 has way more connections.
SB E5 has even two headphone jacks... While it would likely require similar headphones it could surely feed some active speakers.
And still lot cheaper internal Sound Blaster Z with better DAC and output includes optical out for stereo PCM output. (+ AC3/DD or DTS encoded 5.1)
(while new Sound BlasterX AE-5 has very beefy headphone output for sound card)
Sennheiser must be laughing their ass off while igniting cigars with bank notes when counting profits from GSX sales...
Myself not having speakers solved the problem for getting sound also to friend four years ago by warming up ("long" 15 seconds) my soldering station and starting to assemble two Objective2 DIY kits.
DT990 and K712 should make nice similar sound pair for that...