Amplifier Hum - Only Amp Connected

VAC.

Is there any mechanical low buzz/hum emanating from the amp itself, or is it quiet and hum coming purely from the speakers?

Hum is coming only from the speakers and only from the mids as far as I can tell.

What do you recommend for a DC offset issue?

Are the speaker cables anywhere near the power cords?
If so, try moving them as far away as you can.

I don't have a ton of room, since everything is fairly tight. I will try to move things to a different room at least to experiment.

To clarify, hum begins as soon as the soft-start period ends.

The hum sounds like the 120Hz hum in the video in @jea48 's link, albeit perhaps with less sizzle.  Also, I listened again this morning and the hum is slightly louder in the right channel than the left channel.

The amp is supposedly 5 years old.

There absolutely is a hum when the preamp is turned on.  The hum does not change in any way when the preamp is off or on, including when the interconnects between the amp and preamp are removed or connected.

I will try the cheater plug this evening.  Does it make sense that a ground loop would be worse in one channel?

I did not use tape but the ground tab was not in contact with the screw.

Cheater plug did not make a difference.  Hum was still there.

I swapped the power tubes and it is possible the worse side 

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.