Amplifier Hum - Only Amp Connected

This sounds to me like a power supply problem such as a failing filter capacitor in the power supply. Such a part can indeed cause more hum in one channel than the other but the problem would be in both. Sometimes you might be able to get it to stop humming for a little bit by simply turning the amp off and on again, but only sometimes. My recommendation is to seek service on the amplifier.

Thanks for your responses and advice.  Seller asked me to ship it back to them so they can take a look.

Follow up:  I sent the amplifier back to the seller, which sent the following:

"I was able to get the amp on the bench today and test everything out. Everything is working great. The hum that you were hearing is remedied with a slight tweak of the bias to get the pairs of tubes biased correctly. The bias light system can get you very close but very slight adjustment of the bias cleans up all of the remaining noise and gets everything perfect."

The "bias light system" mentioned above is the VAC amp's bias system whereby a light comes off or on while you turn a screw to determine whether bias is correct.  Does it make sense that a slightly incorrect bias could create a hum audible from 8 feet away?

FYI, I bias my current amp all the time with a multimeter and without use of a light system and have had no such issues.

If there are separate bias controls for each power tube then this explanation makes plenty of sense. If the currents are not balanced, the sawtooth waveform in the power supply can be heard in the speaker. When properly balanced (both bias controls set correctly) then the sawtooth (buzz) waveform is cancelled.