Reduction in Speaker Output

I have an amplifier that for some time, has not provided balanced output to both channels at the 12:00 setting.
I must reduce the balance control to 9:00 to hear output from both channels. If I rotate towards 12:00, the right channel overpowers the left channel. If I rotate all the way to the right, the left channel is off, as is should be. If I rotate all the way to the left, the right channel is off, as it should be. Switching speakers produce the same result, so that rules out the speakers being the problem. I cleaned the pot controlling the balance, but to know avail! Please advise.....

SAE TWO A-14 (1982)