Does the subwoofer have the same hum when the input cables have been unplugged? Many input cables use very fine conductors, which often fail. A bad cable will often introduce a hum and prevent a signal from getting through. If the noise is still present without the input cable(s) connected, then repair options are limited. No schematics are available. There's a daughterboard that has been available, but its failure usually causes the system to die (short-circuit). The cost to repair the amplifier may be close to the cost to replace the entire sub.