McIntosh SS Owners Question

Agreed. There should be no hum at all. Have it checked out.

I had a MC7205 which is the same chassis as the 206 Sans 1 channel. I had the same hum and determined that it was the power supply resonating through the shelf. I found some ModSquad rubber feet that eliminated the hum. Those are no longer made but a similar foot such as sorbathane etc may be your solution as well.

I unplug my Power Conditioner and plugged it into a different outlet. Hum gone.

Was the hum coming from the speakers or the amp itself? If the former, you probably have a ground loop issue.

Lots of discussion here on AG if you enter "ground loop" in the search field.

That reminds me I used a PS Audio PC with the ground plug removed. Sorry I had forgotten to mention that, old age memory lapse.