FWIW I used self-adhesive, constrained layer, mass-loaded, automotive panel sound deadening pads on my 511Bs; top, underside and in the upper and lower front bells. A rap on the front lips gives a very low level and dull 'thunk' which has a frequency well below 500Hz. The finish looks good also.
http://www.wurth.co.nz/cat/files/ass...s/page0198.pdf - German product available worldwide. There are similar products from other manufacturers but this one I know worked for me.
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