One of my friends is having trouble with his subs making one big thump when he turns the amp off. Anything he could do to fix it?

 

thats a sign of bad wiring. or even could be a bad amp. the ground connections would be the first thing i check.

 

is the system integrated into the stock system? or at least is he using speaker level inputs on the amp, connected to the rear speakers? If so, its feedback...

when he turns the car off, the amp still has a little power left in it, or the deck takes a second to power down... the rear speakers act as a microphone, and you know what happens when you put a microphone near the speaker... you get FEEDBACK!!

Try turnin the gain down on the amp a little... if this is the case.