That being said, fxsound enhancer is a nice app to change your audio, if you want something like that. I use it to take off the high frequency bite of shots.
Maybe he turned off map noises, like for example t spawn you sometimes hear a music or an airplane flying over or whatever. Idk if its even possible to turn that off btw.
Pros doing a lot of things which don't have any logic behind it: playing in stretched/non FullHD resolutions, unmounting silencer from USP, using left hand for weapon model...
I'd suggest it's just habit of having bassy headphones which needed to be equalized.