My thoughts are the room, and the quality of your power. The room has two identical dimensions, and one that is a multiple. This is very bad news for getting a smooth frequency response. As far as power goes, I'm afraid the only real way to know would be to get a very good power conditioner. Unfortunately, the good ones are expensive. You might want to try to audition (borrow, buy with return privilege, etc) a conditioner to see if this impacts your problem significantly.
Good luck !
Good luck !