I can see why the loud bang and the room full of smoke and white powder. When the left channel blew I simply cut the power leads to it and carried on with the right channel. But that blew also and exactly the same component – a large decoupling capacitor, electrolytic. That was the clue to the problem, I had put 16V electrolytics in instead of 63V. Hadn’t noticed until now. So that was a nice easy fix!