When I was shopping for stereo loudspeakers at a predominately home theater dealer I was surprised when they demoed by turning the lights completely off. I thought it was dishonest in a way, since I never listen like that and I wasn't certain what was going on. I didn't know it was a psycho-acoustic effect I was experiencing.
A free tweak revisited
I am absolutely positive that this subject has been discussed before, but last night as I was listening I was reflecting on how big a difference (for me) turning the lights off and (probably redundantly) closing my eyes makes. My listening room is a quite near field environment, but with the lights off/eyes shut I hear the illusion of a sound stage wider than the boundaries of the room with height and some depth. (However, the sound stage I get is not as deep as it is wide.) Opening my eyes and looking at the speakers so close to me is kind of like sticking a pin in the balloon.