These are difficult to work on, everything is packed in there. They also have unobtainable dual filter capacitors which have to be replaced with 4 single types if they fail, there are several threads on this, I don't consider this a deal breaker, perhaps they are the source of the hum, perhaps not. These also have an unobtainable toroidal power transformer. If the unit powers up and plays, that should be OK. I would make sure it has the proper fuse in the rear panel to protect the transformer.
Tom