Clean it, get the bias adjusted, and measure the power caps.
My amp is 50 years old, still 100% stock inside.
Amp showing its age?
Before getting into details, just the bottom line: can an SS amp, well into its second decade, begin raising its output impedance and/or dropping its peak current capability? Because mine certainly seemed to be doing one or both, so I have just replaced it, and now the speakers are again obedient.