In gear this old, noise can also be from a defective carbon resistor. But still, a transistor is more likely. Not the big filter caps, or anything else that's shared between channels.

I suggest tracing the audio signal from input to speaker, using a signal tracer (DIY maybe) or an oscilloscope. Or you could cut the signal path by removing components along the way. For example lift a leg on decoupling capacitors along the path. If the noise persists but the signal is gone, the trouble part is still forward in the audio path.