Tom, the only thing that has a touch of madness about it is doing all this figuring without having a precise measurement of the vertical space available. You mention 8.5", but I made a quick comparison(hardly precise)with the VP100 height, and figure it's at least 11". The regular M2 might just be able to be squeezed in. If so, that should be the pick; it's essentially the upper section of the M60 and should match very well. A couple of years ago Dean tested extensively and praised the M2 as either a center or width speaker with the towers.
If it can't possibly fit, my next thought would be that the in-wall part of the M2 in/on certainly would.
