Board repair for subwoofer

You can find plate amplifiers from Dayton at Parts Express or fancier Hypex models at Madisound and miniDSP.  besides physical fit and power I encourage you to get something with good DSP capabilities so you can properly tune the sub.  If you are inexperienced, stay away from Hypex as their software is a PITA and stick to Dyaton or miniDSP. 

Im sure you mever use the 800W.  A Hypex 502 would give you 2x 500w if each is 4 ohms.  Plenty.

@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! 

I think Crown makes some professional amps with built in DSP and crossovers?  If so, that's ideal.  Almost every sub benefits from parametric EQ. 

@devinplombier 

It seems that most subwoofers are using class D amplifiers these days:  is that because it is better, or because it is cheaper?  Would you recommend the class AB over the class D for subwoofers? (Hoping not to open a can of worms.)