Damping to exitation of parasitic modes of

What is the best material to use as a substrate for making Isolation plinths/platforms, for damping the excitation of parasitic modes of vibration as well as HF resonance? That looks cosmetically appealing as well?
Well jollygreenaudiophile, I'm not sure if this is what you mean, but I've had great results with Soundcoat adhesive sheets. I have it stuck on all my electronic chassis's as well as on the top & bottom of the upper platform (which the TT sits on) of my rack. You can get it at parts connexion. Hope that helps.  
How well does it work? I'm looking to augment some Maple.
And thanks!
It works great on the 1/2" steel plate platform my TT sits on. It's now the best of both worlds, ridged & dampened. There was less of an improvement (though a good improvement nonetheless) with the electronic chassis's. It's pretty cheap, around $12 a square foot
forget everything except altering the Q - you will first need to determine the freq.s involved (tho any decent car stereo dampening material should be helpful)
I've tryed using sorbothane but have had problems with the substance degrading. It also ruins wood finishes!
For micro vibrations best to use constrained layer damping as well as very hard cones that will allow such unpleasantries to escape the system. I also am a fan of natural cork for damping and isolating printed circuit boards, capacitors, those pesky large transformers and gulp, fuses/fuse holders. Sorbothane not so much. I've had trouble with the sound degrading. 😛

+1 on constrained layer damping and not Sorbothane.