Not sure about that model, but if I remember correctly, that power amp board has a circuit ground that only connects with the three pin Molex plug.
If that is the case, then your problem may still be in the power amp, but it is being hidden by the fact that you are disconnecting the ground from the circuit.
I'd suggest that you go back to square one and retest all of the transistors and diodes on the power amp board. You might want to build or use a light bulb limiter on this one to save yourself a lot of trouble.