I had same Problem with the first Stanmore I, after 5 Years, no sound any more. Luckily, a Colleague is a really good Electronic Freak, he tried to repair it. He locate broken Chips, what was burned, because of to much heat. He replaced it, but mainboard burned again. He told me that the electronics on the boards get to hot for the low Quality Parts. (By Design, maybe that’s why Marshall came out now with the Stanmore II)
Anyway, i bought a new Stanmore I and will put on the back Aluminium Radiator fins and hope that will solve the overheating Problems.