1 channel louder than the other on Sansui 719

I recently purchased for a good price one Sansui 719 amplifier.

The amplifier sounds nicely, but the left channel sounds attenuated compared to the right one, expecially at the mid-low frequencies according to my hear. I haven't taken any measurement of the frequency response yet, but I am planning to do it soon.

I was planning to apply a sinuisoidal signal with the same signal to both channels and to measure the amplitude of the signal at different points of the signal path, in order to identify where the attenuation happens.

Before doing this resource consuming activity, does anyone has any idea if there is any I should focus on first?

As a first step I was thinking about checking the balance potentiometer and the input stage.

Any suggestion?
Is the same problem apparent with headphones?
Yes --> Fiddle with tone, loudness and attenuator switches, those may require cleaning. Possibly some of the pots, too.

No --> Does headphone sound change when connecting and selecting speakers (a mute dummy load or well-isolating headphones would work best here for obvious reasons)?
* Yes --> Sounds like the protection relay will need cleaning / replacement.
* No --> Speaker selector switch may require cleaning.

All of these issues are pretty common in 35-year-old amps. Along with the odd dead electrolytic, of course.