@drbond Were I in your situation, I would abandon the original plate amp in place, redirect the speaker leads from the old amp to new binding posts or Speakon connectors - essentially turning your sub into a passive sub - and drive it with an external pro amp, something in the 500wpc range and configured in parallel mode. Parallel mode takes a single input signal and feeds it to both channels of the stereo power amp, acting as an internal "Y-cable" if you will, thus allowing one mono source to drive both speaker loads independently.
Crown makes several models that would be perfect for this application. Their vintage Macrotech series amps are legendary Class AB monsters with a damping factor of 1,000, and their modern Class D amps have tons of punch and excel on bass duty. If you do decide to go that route, I think you’ll find bass tightness and slam have pleasantly improved. Good luck!

