Have you tried hooking your 1215 up to the sub out on the 950 to rule out the possibility that the 1500 isn't just blown. You mentioned that you used a Y connector to pull signal from the main output. When you tried that did the mains have the same popping as the sub? That would tell you if there was a signal problem.

Also feed one of your front mains with the sub output and listen for the popping from that output