The rear deck is a two layer stamped steel piece, it's the plastic mounts and top cover moving against it that buzz. At this point probably going to fabricate proper MDF mounts for the front and rear, and sound deaden the doors and rear deck. For now I'm going to throw in the matching set of MM691's with the MM6501's. They received great reviews all over the web and now with proper mounting/ sound deadening they should do the trick. I can obviously add a sub stage but with the amount I wanna spend and results I wanna see this choice makes the most sense right now. Any input?