hi ibuy,
it is hard to determine the exact cause. it could maybe any of the resistor in your preamp that has drifted in value. it would be best to send it back to manufacturer to check out. However, if the imbalance is near the lower volume settings then it is definitely the potentiometer problem, because generally potentiomenter are poor in channel balance at lower volume setting unless you purchase those costly potentiometer.
it is hard to determine the exact cause. it could maybe any of the resistor in your preamp that has drifted in value. it would be best to send it back to manufacturer to check out. However, if the imbalance is near the lower volume settings then it is definitely the potentiometer problem, because generally potentiomenter are poor in channel balance at lower volume setting unless you purchase those costly potentiometer.