I removed the lower woofer, I would say I diagnosed the problem correctly. With it disconnected the bad sound stops.

Looking at it closely I don't see any obvious rip/tears or anti-matter seepage. It just moves wrong. Violent shaking which causes the rattling/buzzing sound. (and yes, the polarity is correct).

So as soon as the replacement arrives this one goes back home to the mother-ship.

M22's x 3, QS8's x 4
SVS PC12-Plus, Denon 3311ci
Ear-to-ear grin? Priceless!