There is a guy in Perth that specializes in JL audio
The hum issuse was quite common and I think there was some mods that were required
I know that Perth is a long way away but
1 You could unscrew the plate amp and just send that
2 Ring him and see if he has a contact in NSW that could help you
RF Waves 08 93427161
His name is Chamila
He is an excellent Tech
Cheers