Welcome back.
Visual inspection of electronic parts is a good thing to do, but not all bad parts will show signs of burning, leaking, etc.
If I recall correctly, the amp would sometimes work but usually just hummed and the speaker cone would extend and stay there. These are signs that there is dc voltage on the speaker and that there is some intermittent connection that is causing it.
You have visually inspected all of the components and the wiring and have not found anything that looked questionable.
Did you ever try flexing, tapping or knocking on the power amp board to see if you could either cause the problem to start or to stop from a mechanical stimulus? Often doing that will point you in a direction to investigate for a cold solder joint or broken component lead, etc.
If the problem does not respond to a physical movement, you will have to move on to testing parts and wires, etc.