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.
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.