id put my money on transistors...
I once has a QSC Pa amplifier, which used to trip one channel out past a certain volume..
Opened her up, weakened and damaged solder joint, giving intimant connections on the PCB, causing the Transistor to blow...
In a sub, it could also be a possibilty, especially with all that air pushing inside the cabinet, can quite easily help damage a solder connection over time of heating and cooling...
But +1 to the dimming bulb to troubleshooting.. i used either a 35w or a 20w bulb, concidering what im working on, and a ACVAC, variable voltage regulator, so i can run amps at lower voltages while troubleshooting, and then slowly turning it up to full power slowly, making sure everything is ok..