Subwoofer plate amps are one of the worst things to try and repair. Nobody offers service manuals because they are not designed to be serviced. Due to the amount of air flow and vibration, many of the components are covered in epoxy or resign, so they can not be easily diagnosed or replaced. The circuitry gets updated and redesigned often, so the production runs are not long and servicing is rarely a consideration. I have a similar popping problem in a Klipsch sub and they could not provide a replacement plate amp or even a circuit diagram. These are basically disposable devices that you trash when they break. Replacement plate amps from PartExpress will not have the exact same frequency response designed for your specific speaker, so they are not a viable option. Best bet is to find a service tech with an oscilloscope or proper signal generators and tracers to diagnose the issue without the need of a schematic. The amount of hours involved makes this cost prohibitive so it is only worth it for higher end subs. I used to repair audio gear for several years and these non-servicable designs are very frustrating.