Also a spray can of freeze (e.g. CRC Freeze Spray) is useful, and a jeweller's loupe (x20 magnification or so). If your phone takes good macro video, that can be really useful to look at several times (in the absence of a stereoscopic microscope).
Contacts are the first thing to suspect with any fault. Check inside the plugs on the cables.
For intermitttently disappearing and reappearing bass, though: probably the contacts are OK. Inspect (and if possible, re-make) solder joints in this part of the circuit - you may have a hidden dry joint somewhere. Then old electrolytic capacitors, especially tantalum ones, are sources of trouble.
Of course if the amp has been in a house that's been hit by lightning, or where an arc welder has been used regularly, then the possibilities widen.
If you get a crackling noise that changes when you change the volume, the volume potentiometer either needs to be cleaned or replaced. Similarly if the volume knob has ranges where the sound drops out entirely.
If one channel doesn't work at all, you trace the signal path starting at the source.
If nothing works, look for a fuse (and look for something that might have caused a short).