Harvey had a tip about sanding the inside lip of the bottom shell to make a better ground connection. My v67M didn't hum, but being the curious person I am, I took it apart and lightly sanded some black paint off of the inside lip and low and behold, under the black was a very recognizable shade of GREEN. This solved the mystery in my mind! BTW, I took the Mars ad to a local pro sound/music store and he reluctantly sold me his green&gold v67 that was $225 in his display case for $99! Oh yeah, they do sound exactly the same to my shot out ears.