Difficult to say what the cause of the hum is until you can get it on the bench, from what you describe it looks like an single ended class A output stage, if by chance the output transformer is shot you should be able to get it rewound or find a reasonable substitute, whichever way I would say the amp is repairable.
Lawrence. [EDIT: Missed Alans post]
Lawrence. [EDIT: Missed Alans post]
