If you are monitoring the sound that is coming into audio input C, then selecting INCD to be “C” for the Thru machine (confusingly, the options are only described on page 50 of the OT manual) should give an even stereo signal.
If the left and right signal levels are not equal, you first need to check your setting for BAL on the AMP MAIN page. If there is still a problem, you need to check the rest of your monitoring path.