Haven't noticed it on mine but you can try buying a balanced 12ax7 tube for the V5 position. This may very well solve the issue for you .In a 12ax7 tube, there are two triodes. With the triaxis's v5 tube , one triode feeds the A output and the other triode feeds the B output. So by using a balanced tube in the V5 socket, you get an even output . If it isn't the tube, it's possible that there is a tolerance issue between the two output pots.
You could put a multimeter on each pot and test it to see if the resistance is the same .