FWIW, I bought a McIntosh preamp here on Audiogon that had a slight, but audible, hum that did not affect the speakers. Someone on this forum suggested that I replace the hard feet with sorbothane ones, which helped a little. I noticed that when I lifted the preamp the sound stopped completely, so I loosened the screws that hold the cover down. Surprisingly, it is now silent. Can't hurt to try it!