Magnepan's MDF frame can be damped by Noisetek barrier mats.  I just added some 4mm(⅛") damping to my SMGb's and there is no vibration anymore.  I glued two 102cm(40") long × 6cm(2⅜") wide, and one 38cm(15") × 6cm(2⅜") with contact cement to the back side of my Maggies.
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Above shows voice coil repair done, and on the left is my tweak to minimize diffraction.  Below is another view.  It's made of triangle wooden sticks, glued and painted.  5mm(⅛") × 15mm(⅝") is the size of the sticks.  This will fit nicely to the edge of the frame.
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