I recently started having some trouble with my Peavey Mark III Musician bass head. It works wonderfully at practice, where we utilize a friend's Carvin 2x12 cabinet. When we play out shows, we usually borrow the headlining band's cabinet and use my head... which I'm ashamed of, but our vocalist won't stop booking shows. Anyway, whenever I try my head on someone else's cabinet, it produces an unbearable piercing screech, in which most of the people running the show (it's usually DIY, so we or another band typically handle all the sound setup) demand that I swap the head, so all the time I spent perfecting the sound and tinkering with the graphic equalizer was a waste because I'm using another bassist's head. What could be causing such a screech?