Have you only had this problem playing in the same space? If so, it sounds like a standing wave kinda thing(boomy with odd order harmonics.) Similar to problems thay have setting up subwoofers. It may very well be the room and not the guitar or amp. I'd have someone play with the EQ and gain, while you're creating the problem. That will give you an idea what freq. and amplitude the problem is in. Good Luck!