How much rotation of the volume control are you able to use? If you have a very sensitive, high gain system, it may be that you are only using 5-10 degrees of the pot rotation, and that would account for what would normally be a bit of low volume channel imbalance.
Otherwise, try swapping the tubes to see if one of them may be weak. (Also try swapping D-cells for that matter). A weak tube, however, would exhibit voltage issues that would be quite noticeable (high plate voltage, low cathode voltage).
With your McIntosh off, measure the resistance between the chassis of the amplifier and the shell of its input RCA jack. (This should be very low impedance)