I have an OCM 500, the older sister to the Belles 150. It used to buzz a bit when I had it plugged into a different outlet. I also noticed the buzz would change as certain other things in the room would power on or off (e.g. a vacuum cleaner).
I eventually plugged the amp into the amp outlet on the same Monster AVS 2000 as all of my other components (which are plugged into an HTPS 7000) and the buzzing went away. I know what is said about not plugging amps into power conditioning which is why I originally had it straight to wall on a separate plug, but the buzzing was too annoying for me. I don't know if it was specifically the AVS 2000 that made the buzzing stop or (effectively) putting it on the same wall plug as everything else. In either case, I'm happy with it now.
I eventually plugged the amp into the amp outlet on the same Monster AVS 2000 as all of my other components (which are plugged into an HTPS 7000) and the buzzing went away. I know what is said about not plugging amps into power conditioning which is why I originally had it straight to wall on a separate plug, but the buzzing was too annoying for me. I don't know if it was specifically the AVS 2000 that made the buzzing stop or (effectively) putting it on the same wall plug as everything else. In either case, I'm happy with it now.