Bad solder joints have sometimes occurred in my amp, probably because I use point-to-point wiring, I'm always making changes and the thing has become a bit of a rat's nest over time. Sometimes, what seemed a perfectly sound joint when I tested it can end up parting company and becoming just a contact, giving the sort of problem you mention. However, whenever that happens I can usually detect it by thumping the chassis, resulting in a crackle or worse, depending on which part of the circuit is affected.