Definitely the plate amp. We have a project with 3 of these. 1 failed a couple years ago. Triad replaced it under warranty with the newer model i posted above.

Today i swapped woofers in the two remaining subs. Problem didn't move. Then i tried moving the known sub to the problem area and it worked fine so that rules out AVR & cabling.

It would be nice to just replace the amp and keep the sub because the cabinet its in has the bottom cut out so it can sit on the floor. If i cant find a sub that fits i have to get my hands dirty like a common wiremunky :).