Subs going off the hook (by 3 users)

I will agree with the first solution posted but you can also start checking values with an ohm/volt meter, a digimeter, I would start by checking values of the MOSFET Transistors, If you get more than a couple of bad reading then you got more of a problem than its worth to try to repair it unless you know your stuff in this field, but if you only got like one bad valued transistor you can isolate this circuit and see what happened to make this transistor fry like bacon then replace those parts as well
I will give it an 80% chance that you need new amp but what do you have to lose, it doesnt work anyways :0)