It certainly sounds like a common case of what you see/hear/smell when a subwoofer amplifier fails or is beginning to fail. The popping followed by constant buzzing is a common sign of this. Usually caused by some sort of component failure. With component failures it can also be popping sounds, following a buzz and losing all power to the sub. Could even be a issue with the DSP in the amp which is causing some of the intermittent problems. In either case you need a new amplifier. Also the intermittent helicopter effect you describe is also an amplifier issue. You should not be experiencing this problem either.
