very low level hum and reduced volume from one channel

I built a ST-120 about a year ago and it has been performing very well. I recently moved it to drive more efficient speakers and have noticed a very very low level hum and slightly reduced output from the right channel. The reduced output has been evident for some time and is easily compensated with adjusting preamp balance, however, the hum was not apparent. I am just hoping for a little guidance as trouble shoot this. Thank you.

The hum may have always been there but not noticeable due to the speaker efficiency. When I had a similar issue the problem turned out to be the center driver tube on the VTA board. Have you tried a different tube in that position? If you don't a spare you could move the left or right tube to the center and see if it helps.

I have rolled through various different driver tubes and rotated output tubes without effect. A little more info, the hum does not change with adjustment of the stepped attenuator and is constant 60 hz.

To aid debug you can try an old guitar amp repair trick of desperation by pulling the center tube, turning on the amp, and listening.  Does the amount of hum change?  Same with the phase inverters.  Helps isolate which stage is contributing to the issue.  I don't think there would be any harm in trying briefly other than perhaps tweaking bias to compensate for load changes on the B+ supply.

But speaker efficiency may be letting you hear everything more clearly.