Channel Imbalance at low volumes

Amplifier at this age (30 years old?) all connectors, switches, selectors and pots should be cleaned by using DeOxit

This is due to the aging potentiometer that is used for the volume control.  The volume knob actually controls a two-channel variable potentiometer (one for left channel, one for right channel).  This unit is about 30 years old, so there are likely things that will fail on it or be worn out.  The "variable resistance" surface inside the "left" potentiometer is likely breaking down at the area where the volume is low and is actually showing a higher resistance than normal.  This is very similar to resistors wearing out -- the resistance value starts to rise greatly.  Pretty much the only thing you can do is have the potentiometer replaced.  If you did this, it would probably be worth it to have as much electrolytic capacitors replaced as possible (or as much as you are willing to pay for).  This will greatly increase the sound quality of the unit, as the caps are very old and dried out.

In some case, the output relay is the culprit

I had a Rega Brio with this problem and assumed it was the volume pot. Had a new pot put in and it didn't fix it. God knows what the problem was but, presumably, there are many things that can cause channel imbalance at low levels. Maybe get a decent technician to check it out.

It can be some resistor along the way (maybe on the input of a gain circuit).  With the volume down low, the imbalance ratio between left/right resistance could be far enough apart.   With the volume higher or wide open, it is not such a difference.