There could be something wrong with the amplifier or you could have a ground loop. Try disconnecting everything from it but the speakers and then try using BT. If the hum is gone then a ground loop is the problem. http/web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf
If not then the problem is in the amp. Hope it's still under warranty. Might be an expensive repair.