Hardly a new idea, but it works.
I noticed a low level hum coming out of my "one off", from the unit itself, when no music's playing(not thru the speakers) . Hum could be heard at about 6 inches from the unit.
I took out the power Xformer mounting screws, use the vibration control material I have on hand and made a couple of washers. Installed the washers between the Xformer and the chassis, tightened up the screw, not overtightened. Now the hum noise is all gone.
Music has more clarity, and more detail bass too, verified and confirmed with familiar tracks.