The balance works fine. To be more precise: If I balance the input to the volume control with the small pots you can access from the underside (using the phono pots) so that the two channels are equal when the volume slider is at 3. Then if I move the volume slider to 4 the left speaker becomes much louder. Then if I move the slider further up the scale to 5 the balance at first becomes equal but at 6 the right speaker becomes a little louder. And so on!
The volume control has a lot of connections so perhaps the problem could be elswhere?