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.
FWIW, i have an old sp10 with banana jacks on the back of the pre-amp which must have a jumper connecting them in normal use "otherwise, severe hum and/or oscillation will occur". i don't know if these exist on the sp9 but check it out. if you do, the jumpers may be in place and all that you need to do is to tighten them up. hope thats it.
Thanks, but the MKII doesn't have the jumpers on the rear...
It could be what Newbee suggested. I have the SP-8 and it has the banana jacks at the back that should be tightened. If it's not that. Check all your tubes.
Easiest answer? Call Leonard at Audio Research. Get it from the source.
I agree with checking tubes. Are they OK and are they seated properly in the socket?
I think I found the problem:
I took the cover off to reseat the tubes and found that the large 100 uf 450v capacitor on the pc board had broken off from it's solder point on one side. It must of occured during shipping as it was delivered yesterday and happened immediately on powering it up for the first time. I've contacted the seller to keep them up to date on the situation. Thanks for all your help folks
I sent the amp off to ARC and two weeks later, returned to me with problem solved (broken solder joint on a choke), they installed a new choke and capacitor as well as cleaning all the controls and jacks. 70.00 including return shipping and insurance.