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.
Did you try a simple cheater plug on the Sutherland power cord. I know that that you did try a Hum X but that is not as simple as a cheater. I used one succesfully on a CDP which also introduced a 'buzz'. 
Don't know if you are grounding phono stage or turntable to the receiver but his is from the manual.......

The earth terminal (SIGNAL GND) of this unit is not for safety grounding purposes. If this terminal is connected when there is a lot of noise, the noise can be reduced. Note that depending on the turntable, connecting the ground line may have the reverse effect of increasing noise. In this case, it is not necessary to connect the ground line.

When you say "turn the volume to 50," are you talking about halfway on the volume control?  What level do you usually set it when you're listening?  
Just out of curiosity is the turntable input on your Marantz a true phono RIAA input? If so your running your table into a phono stage and then into another phono stage. Try running it into one of your line inputs.
Good question about the turntable input.  I am using just a regular line input with the Sutherland stage.  Thanks for your response.

Try this
Scrape the paint off the motor point you want to connect a ground wire to. Connect a wire from that point to your phono stage/preamp etc.
I've had quite a few different phono stages, and I don't think I've ever had a setup that didn't have noise around halfway up the volume dial.  The high gain of a phono stage can do that.  Do you listen at 50 or above on the volume?  I never worry about it unless the buzz starts in my comfortable listening range.  
Your cartridge is a moving magnet and your turntable should not be ran thru the phono input on your receiver with the phone stage. Assuming you run it thru one of the other line inputs,confirm that you have the correct loading & gain shunts in the correct position in the Sutherland for your cartridge. If all else fails, shoot Ron Sutherland an email. He may be able to advise or help you correct the problem.   
@motoraway - he is actually using a Sutherland Insight phono stage which turns the audio signal into a normal level for the Marantz.

I don't have direct experience with phono stages and buzz, but if it's a ground loop, this appears to be an alternate solution to cheater plug:

Try this:
Turn the volume to above 50 without playing a record, move the Sutherland Insight away form other components and see if the buzz reduced.
I'm grounding from the turntable to the Sutherland preamp/stage at present
Another thing you can try is connect the ground post terminal of the Sutherland Insight and the Marantz SR7012 signal ground post via a short wire.