Hearing another source that’s still on isn’t that uncommon.
On the other issue i agree that it’s most likely a dirty input selector, my suggestion would be until you can have all the controls cleaned and lubed exercise them a bit. By this I mean, turn the amp off so no transient thumps, then work all of the controls several dozen times each. For example, rotate the balance knob clockwise then counterclockwise back and forth rapidly over and over. This will wipe away most any tarnish that may have built up on the contacts and usually alleviates most issues,for a time. This in no way replaces a good cleaning, but should help temporarily.
Martin