I've always liked Marantz for audio. Just hooked up a new system with a NR1710 two days ago, replacing a NR1605. And about a week ago, finally got my older AV8802 system going with newer HDR equipment (since the AV8802 doesn't support HDR). Always love the sound, and control is nice and predictable.

As for the problem... have you swapped the wiring of the non-buzzing speakers to buzzing ones to see if the buzz follows the wiring or the output on the receiver? If the output, have you simplified to just receiver, power into the wall, and 2 of the buzzing speakers, nothing else? If that still doesn't work, seems like the only final thing to test is a new receiver.

Also, Marantz receivers typically don't have a separate ground prong on the cord, but they do have an optional ground connection on the chassis.