A true balanced preamplifier, such as the Audio Research LS 5, uses a four-gang selector switch and a four-gang volume control. Any problems with either of these would cause a volume imbalance.
Have you switched the balanced interconnects to determine that one leg of the connection has not failed? This too can cause one channel to play louder than the other, assuming it is not your speaker or power amp.
You did not state, but you did have good channel balance until recently?
Have you switched the balanced interconnects to determine that one leg of the connection has not failed? This too can cause one channel to play louder than the other, assuming it is not your speaker or power amp.
You did not state, but you did have good channel balance until recently?