TLS Guy, thanks for the advice. I've reached out to PSB inquiring about a replacement output board. I confirmed that the stereo produces a tone with a different sub, so it's definitely not a problem at the input.
The problem is that you don't actually know the problem is with the output board. Usually with those class D amps it is the switching power supplies that cause trouble more often than not.
Don't forget that a power supply has multiple sections with different voltages and current outputs. Low voltage for the control and voltage amp stages and higher voltages for the output stage.
You need a circuit with voltages marked at the least and preferably a service manual as well.
You also need a high impedance FET VOM as using a cheap multimeter can do damage and give misleading results. Then you need to confirm the stage where the problem starts by tracing a signal from a signal generator and tracing through where the signal gets interrupted.
There are no short cuts to service work and you have to have some experience the right equipment and at least a circuit with test voltage points marked.
So in your case you need the whole amp unit replaced.
I have been down these type of situations with posters before, and so often these type of repairs lead to failure and a waste of money.
After writing this post I see from searches that this amp is a horror story. It is a diabolical contraption that continuously varies the power supply voltage with load. Experienced service people on the DIY forum have had no luck at all trying to repair these.
I have found that PSB will not service them or provide parts only replace the whole amp, which I understand is quite expensive.
These people have ended up cutting their losses and just getting an external amp, usually Crown, and fitting speaker terminals on the back of the sub. Since this is a ported unit, any good external amp of around 200 watts will do the trick.
Realistically your options are to send that sub to the recycling center, or adapt it for an external amp.
Quite honestly the moniker Not Always Dependable is deserved.