Channel Imbalance at low volumes

Hi guys!

I got a sweet sounding NAD 7175PE the other day! It sounds rad at high volumes, however, at low volumes a significant channel imbalance appears. The left channel becomes MUCH dimmer than the right. The problem exists while using either the A or B terminals, but it also flips sides if I swap the left and right speakers, so it's not the speakers or speaker wires. Interestingly, the "A+B" speaker selection produces no sound for some reason. Not sure if that gives any clue as to the cause. Anyone know what's wrong? Thanks!