I got my HE400SEs now. It’s my first time having an open back headphone and a planar one too.
For gaming:
My USB DAC just barely powers it enough to hear enemy footsteps. I need to upgrade my setup soon to fully utilize it. Everything sounds and feels natural and no fatigue for 3 to 4 hours of competitive play. The imaging and soundstage is good enough to match my IEMs but for FPS, specially tactical shooters I wouldn’t recommend it unless you have a good DAC/amp to power this bad boys or else you would for sure miss someone due to not hearing their footsteps. For open world or any RPG, it’s a nice headphone.
For music/movie:
I have not qualms about it. Layering of instruments is amazing! For movie or watching content, I was amazed the first time I put it on. I was watching LeBron’s Drew League game and I felt I was there haha!