Materials to reduce sidewall flex & vibration ?

FYI. "Epoxy granite material has an internal damping factor up to ten times better than cast iron, up to three times better than natural granite, and up to thirty times better than steel fabricated structure." Answer found in other forums.  

Thanks to bdp24 and meerzistar for contributing to reducing side wall flex and vibration.

I'm still researching;here's new info..
"  Hardinge as well as many of these; others are quick to point out that the damping properties of epoxy granite are about 8x those of cast iron."  I'll be looking to confirm this as it appears very relevant. 

Well the gold standard is carbon fiber. Expensive and hard to find it what it does is absorb the back wave  and convert that energy into heat. Saving that I would look at bitumen, or modified bitumen. They are often sold as a water proofing membrane that is played under shingles. Soft and sticky it can be cut to size and attached to the inside of your cabinets. 

Maybe ceramic tiles?  Rigid, kind of light, and pretty inexpensive to try.
 
And the speakers would look right at home in the bathroom!