You must isolate the circuit board to heat-sink to keep from short circuit. May be your power transistors are on the main pcb, thats why it is shorting out everytime.
Because of applying higher input voltage to the transistor than specified by the manufacturer. for example, if my stereo power transistor's specification has input voltage defined as 10V with 10 percent tolerance, then for correct output ,the input must fall between 9V to 11V. If applied voltage is more than 11V, the transistor is supposed to be shorting out.
Either you have too much power going through the stereo or you wires are getting old and can't handle the power they need to. You should try new wires first to see if that will fix it.