[Arran]

I don't think that the sockets are the problem, unless the leakage is from the plate pin of one of the #45s to ground in some way. I would try a substitute bias resistor in place of the one in the big voltage divider, maybe its a problem that only shows up under load? Since the bias section goes from the center tap of the #45 filament winding to ground it's easy to try. What about the output transformer itself? Shorted primary? Leakage from primary to the core? It could also be that the set doesn't like the substitute audio interstage transformer, normally they are a 1:3 ratio but back then the factory could have had one custom wound.
Regards
Arran