One of the suggestions never occurred to me, which was to check that the binding posts were not loose. That was not the case but it never crossed my mind to check something so obvious.
The other was to unscrew the amplifier section and look for anything loose inside. I never got that far because, after accidentally bumping the cabinet with my knee in the process, I noticed that one of the vent ports was the cause of the vibration.
The Outlaws provided me some directions on how to apply adhesive but also offered to do the work for me. I am not the handiest person, so I opted to ship the sub for repair.