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.
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: