I wish I had a better setup for electronics repair. Still missing a few important pieces of diagnostic equipment. I still follow a few channels on Youtube that do electronics repair and I think those plate amps are serviceable as long as there is not too much of that horrid glue poured over the components. You can break the circuit down to a switch mode power supply, pre-amp (which includes the crossover and filtering) and the amplifier section. As many amps have an auto-on feature, there is also a constant source supply to run the start up circuit. Once you have seen enough of these amps, you develop the ability to identify the various parts of the circuitry even without a schematic.
The majority of failures seem typical: bad capacitors or blown output transistors. Old glue can also become conductive. A single bad cap can cause the switch mode power supply to fail. Repairability also hinges on the use of surface mount components. If any small surface mounted parts become defective, you need a digital microscope to be good at replacement. Resistors, caps and transistors can be replaced with a soldering iron, but surface mount ICs need hot air.
The repair costs can be hard to justify on a $250 sub but for a good quality unit it would be worth investigating. It's getting difficult to find competent repair technicians, though, willing to take on these types of repairs.
