I had this exact same problem, it can indicate that there is an impedance mis-match somewhere. i.e. using 4 or 6 ohm speakers instead of 8. However if you unplug the speakers and it is still doing it then you could have a problem with the actual amp circuit.
Mine was doing it with nothing plugged in and had dry solder joints on the circuit board with the two large output devices are. Simply resoldered those and it is fine now.
It had started crackling before it stopped working all together, which would also indicate the start of dry joints or bad connections somewhere.
on Mar 22, 2015