Oscilloscopes are great, and maybe the most powerful diagnostic tool on the bench. But until you know what you are looking at, there is little point in getting one. You want to learn to work on amps, but you'd have to learn to operate a scope as well. Not to mention learn what a meter is telling you. By all means learn this stuff, we help people every day do just that, but it is not simple like show me how to play solitaire.
DO find some sort of kit to build, that will teach you to solder and deal with parts. Also, when you make a mistake, it is not on your primary equipment, it is on your little kit.
To actually approach your amp, we would want a schematic. I am not personally familiar with that amp. If it were in my shop, I might not need a schematic, because I have many years experience in such repairs. But I cannot direct you to things we have no means to locate.
Off hand I would suspect you have cracked solder connections on the main filter caps, and possibly the jacks and controls along the panel. Not to mention the controls likely need cleaning.