AR SP 9 hum/amp compatibility question - HELP

Just bought an Audio Research SP 9 mkII here on the 'goN.
It arrived yesterday nicely packed in it's double box and looking like new. Bought it to replace a Counterpoint SA 7.1 in my main music system.

Hooked it up to my Sunfire Signature/Magnepan MGIIIa setup, turned the gain all the way down, set the mute and powered it up. It set off a low buzz, or hum, in the maggies even with the mute off that could be heard from 30 feet away. I let it settle in for a few hours but the buzz is still there. If you switch the mute to operate it sets off a speaker frying hum of scary purportions.

Took the SP 9 out of the system and moved it to my second system, exact same thing. Swapped the Counterpoint and a Sunfire Theater Grand II pre/pro in and out of both systems to make sure the problem was not in the rest of the gear. (second system amp is a B&K ST 140)No noise, not even tube hiss from the Counterpoint with the gain up high.

First thought: ground loop in the AR. Tried a cheater plug to lift the ground, different outlets, into line conditioner, out of line conditioner, no help. Left it on all night but the buzz is still there. Cannot risk setting the mute switch to operate again without frying the maggies (yes the hum/buzz is that bad).

Any/all suggestions or advice would be greatly appreciated.
Thanks!