In case you are curious about
Audio Technica ATH-ADG1X vs Sennheiser GSP 600
The short version is: Both are next level compare to A40s.
A visual analogy will be upgrading from 360p to a 1080p resolution.
Open back vs closed back design, so the sound experience is different by definition, is pears vs apples.
ComfortADG1X takes the crown, very lightweight are floating in your head. GSP 600 are heavier and clamps into your head (to isolate you), is not a too strong clamping force, and the earcups material is very comfortable but still gets tire after a while.
General SoundingADG1X are definitely more neutral and natural sounding, it feels more like a monitor headphone, like having a speakers floating on your ears, while can still hear your voice and be aware of your surroundings.
GSP 600 are completely gaming design, bass is rich and present, middles are highlighted and maybe too crisp treble, in general is a more "fun" seasoned isolated sound stage.
Mic PerformanceBoth are perfect for streaming,
GSP 600 are sharp, and
ADG1X a bit bummy, yet clear. This test was using either mic input to a PC or a Sennheiser USB sound card to PS4, with MixAmp everything gets weaken due the noise gate. If you have mic issues (doesn't sound properly) I recommend: Home preset, no side tone to get the most of your mic. On the
GSP 600 however the side tone is necessary so you can hear your voice, but no go further than 70%.
GamingFirst of all both can give you and advantage regardless; hearing footsteps and sound separation(sound imaging), and positioning are outstanding.
ADG1X on stereo (no mixamp) sounds more wide sound stage, it feels very natural due the open back, you can estimate enemy foosteps proximity easier (how far is this guy from me), but the explosions are numb less vivid (lack of bass)
GSP 600 on stereo (no mixamp) it quite amazing how they tune the drivers, even though is closed back and is very isolated experience, you don't feel the sound booming in your head is a bit expanded but yet closed. Positioning is better, due a very sharp stereo separation but you can't estimate the footsteps proximity like on the other headset. Treble is harsh sometimes.
Coupled with Astro MixAmpADG1X can gain a nice bass and mid tones sounds richer. The sound stage is wider, if you were there out of the open. footsteps (mid and high treble) suffers a bit, don't get me wrong are noticeable and very positional, yet subtle and it feels more natural and is less tiresome.
GSP 600 gets a very large sound stage, sound doesn't feel in your head anymore is coming from the walls. footsteps pops up very clear, like a radar, for competitive scenarios I will say these headsets definitely give you and advantage like a spider sense; but after a while this feel like too much input and gets tired at least for me.
CONCLUSIONAt the end It comes more to personal preference, and your scenario.
If you are in a noisy environment, play competitive:
GSP 600 with MixAmp is spider-sense, hands down. For music are great for hip-hop, rock and vocals are very in-front. Action movies sounds great on these.
If you are more in casual gaming and campaign stuff like red dead redemption 2, the witcher, you want more natural ambient noise, go
ADG1X with MixAmp it will be fantastic immersive experience. But if you want to go FPS competitive I highly recommend to try without dolby surround and a juiced bass EQ preset, due its open-back design you don't need artificial widen sound-stage. On stereo sound whoring(footsteps) is greatly improved. Classical music sounds great on these, vocals are a bit back next to the instruments yet present. Horror, suspense movies shines on these cans.
So yeah, lol I though on make a small post sorry but I hope is useful. Also, some audiophile terms are unavoidable, next I will make a small reference what these terms means to our gaming needs.
Some Audiophile terminology translated for Gamers:
Bass = punch, rumble, explosions.
Mid or Midtones = gunshots, footsteps, in-game callouts.
Treble = Adds sparkle, some footsteps, reload sounds, some ambiance like rain.
Neutral or Monitor Headphone. Means flat none of the tones above are predominant, is how the game designers wants you to hear the game, no equalization.
Sound Stage. How you perceive your surroundings, the space and environment of sound, spatial cues.
Sound Separation and imaging. Is the audio distinction between sounds heard through your headphones, results from the placement of sound sources and cues at different points relative to your position. e.g. "I hear an explosion at my right is far away, but someone is coming from my left... -and be able to distinct if is a teammate or enemy-" 
