sounds like its bottoming out and its hitting full excursion.
might be tinsel lead slap though. thats where the little silver strings that go into the center of the speaker that are attached to the speaker terminals hit the underside of the cone.
definetly not bottoming out and its not the tinsels either, it seems like the cone could be seperated. I've had that happen in the past and I sent it in and tehy fixed it for me in like a day.
no idea about the voltage but that can't be it. They've been working fine untill I put this back in the back of my trunk and it could have been hitting the woofer or touching it.
I remember back in the day there were discusion about how the cone seperates from something and there's an easy way to fix it i think