Help! Can't Get Rid Of Buzz in system at Higher Volumes

Help!  I currently have a VPI Prime Scout TT with a Nagaoka MP-500 Cartridge loaded running through a Sutherland Insight stage/preamp into a Marantz SR7012 AV Receiver.   With the turntable input selected on the Marantz, when I turn the volume to around 50 or above without playing a record, I am hearing an annoying buzz that I am unable to get rid of.  This is not a 60Hz hum but more like a 120Hz buzz.  I've tried using a Furman power conditioner strip, a HumX from EBTech ground isolator, using two different power outlets for different components, different phono cables, a variety of turntable-to-stage ground wires, etc.  The noise is not present when using other Marantz inputs.  The only thing I haven't tried is installing a coaxial isolating transformer for the coax cable coming from my DirectTV satellite dish (I doubt if this is the problem as I don't notice the noise when the TV Audio Input is selected) and disconnecting a dimmer switch (on the same wall as the A/C outlets that power my audio system) that controls a resessed light over my fireplace mantle.  Any suggestions as to what else to try?  Thanks much.