Balance different in A B speakers

You should begin by looking at the service manual to see how the switches are configured and whether any relays are in the speaker feed circuit.

Most likely suspects would be poor/tarnished speaker switch contacts and/or tarnished relay contacts.
 
You're welcome 🙂 And if you get stuck then you know where we are.

The fact that (by implication) that the left channel and right channel are both individually OK as each seems to work correctly via different outlets... so you need to look at the path the signal takes from each amplifier output on the board to the outside world.

Good luck.