Probably not too complicated, unless you have a bad transformer - which you can replace too.

Depends on the amp, but that sounds like an issue fairly common to old Pioneer SX7XX receivers, and the solution is to replace the power supply caps that filter the input section. I feel like if it was your main PS caps or a grounding issue with the volume control the hum would be variable with the volume setting. I don't think anything inside that amp is made out of unobtanium so it's fixable and while it might cost a little to fix the amp, the consideration is how much would it cost to get an amp of equal quality, which would probably be much higher.

You might have a bad filter capacitor, or it could be a bad diode in the power supply rectifier section.

After long storage it is not uncommon for one of the large electrolytic power supply filter capacitors to go bad, so that is the most likely culprit. They need to be charged up occasionally or they can go bad.