Best products for baffle material s ?

BTW, Ptss: You mentioned doing the cabinets in black.
Corian has a color called, "Nocturne" that can be
polished to resemble a Piano Black finish. Would look
sharp, with no visible seams and a 1/2" radius, on all the
corners. Personally; I'd dovetail the sides, top and bottom
and install the front and back with dado joints. Talk
about a solid/non resonant cabinet! I can send you a sample
piece, if you want to get an idea of how rigid, dense and
workable it is.

The width of baflle influences more the frequency response of the driver, or its spread?

Magico got back to me and clarified the material used in the front baffle of the S5's is pure 6061 T6 aluminum which is not extruded. Interesting as the finish doesn't feel like aluminium. But today I gave it the knock test & it sure does sound like it! I'd estimate the front baffle is the same, or similar thickness to the 1/2 inch thick side panels (ie: bloody solid).

How about 3/4 or 1&1/2 inch black granite countertop green glued onto 3 inches thickness of MDF? Very non-resonant?

I have a pair of concept 90 CM1/CM2. The cabinet for the CM1 is a phenolic material and is completely braced in a matrix (the original matrix speaker). The speaker cabinet sounds inert with the unscientific knuckle rap test. I don't recall what the original baffle material is. Is it phenolic, and how thick is it? I know the cabinet must be completely air tight as any air leak causes the woofers to bottom out. I can't see how you could gain much by changing the baffle material.
Granite rings and really is not a sensible choice in my estimation.