Any suggestion why it went?
These amps are notorious for problems as regards heat, because the valves are mounted horizontally across the chassis and the heat just goes straight up into the circuit board.
The tech thinks it went because the amp might've been running on only 1 EL34 for a while, which may've caused the damage to the output transformer.
I'm not sure this is true though, as I remember looking in the back of the amp and seeing both power valves glowing.
I'm a bit of a dumb dumb when it comes to the inner workings of amps though, so what do I know :D