Ah, the Elite 800's! Those are nice!
The thing you want to turn off is the DTS Headphone X Surround Sound, so that your headphones are in stereo mode. You can do that by holding down the Preset button to cycle surround modes until it tells you that you’re in Surround Off mode.
The Xbox Console will do the surround processing and pass that audio to your headset through the wireless base station.
It looks like while you’re in the surround off mode, you can tap the preset button to select from different EQ settings. Feel free to use all of those, it’s just the surround sound itself you want off in the headphones.
I hope that helps you out!
I haven’t owned a pair of Elite 800’s myself for a couple of years, so I cheated and looked at the manual here, haha.
Thanks for reading and the question!