Continue testing with 1 kHz sine wave input and with loudspeaker connected. Put a 470 μF - 1000 μF capacitor in series with the loudspeaker to protect the loudspeaker. Keep the VOLUME control low to show moderate distortion.

Show the oscilloscope signals at the base and emitter of Q504 and Q506.
Test at resistors R510, R512, R514 if these are safer to reach.

Were Q504 and Q506 ever replaced?

Q504 2SC3944
Q506 2SA1535

With the power off,
check resistors R516, R518, R566, R568 in-circuit (all 2.2 Ω)
check resistors R548, R550, R570, R572 in-circuit (all 0.22 Ω)

 

No, Q504 and Q506 were not replaced. I removed them from the PCB and tested them with a component tester, and they passed. the tester is below,

However, since the test current on these multi-function component testers is extremely low, could be sometimes give a false positive under actual circuit conditions, so I will try to replace if I have spares that can be fit.
Update will come.
Thanks