Isolation for Granite Surface Plate

@oldskoolmark - An approach I use to prevent ringing is to employ a "sandwich" of differing materials.

• My rack has a steel frame.
• Between the frame and the MDF shelf is a thin layer of Sorbothane on the frame struts
• between the MDF and the granite is a layer of foamed shelf liner
• my components all have brass cones to drain unwanted vibrations into the granite/foam/mdf sanwhich

See details in this link
Isolation | My Audio Alchemy (image99.net)

• The sorbothane between the rack and the MDF shelf prevents the frame from vibrating
• The foam drawer liner prevents both the shelf and the granite from vibrating and
• the different densities of the MDF and Granite combined is quite heavy
• This minimizes movement due to soundwaves of the music
• The approach , makes for a very "inert" and dense mount

I originally used an MDF plinth on top of the MDF shelf, but found using a significantly more dense material like granite for the plinth worked much better and the cones do not dig into the granite like they did with MDF, which anchored the foot to the shelf, which is something to be avoided

I have not found anything better to date

Regards - Steve

Thanks guys for the benefit of your experience and research! The sorbothane / MDF / foam shelf liner / granite slab approach appeals to me from both cost and aesthetics perspectives. 

Are all MDF boards created equal? If not, what are the relevant quality parameters?

You're welcome but to be honest you don't get the benefits unless you follow the advice. The MDF idea you seem to like is so far down the list of things that work I never even mentioned it. Tried it, crap, moved on. If you don't want the best I get it, they're expensive. That's why we recommend Nobsound. If you want to reject both of the two best ideas and move way on down the list to stuff I rejected 30 years ago, be my guest. Just don't thank me while saying thanks but no thanks. 

Seriously. You don't even know anything about MDF, but why would I explain when you've already demonstrated such a knack for rejecting solid info?

oldschoolmark -  Here are some links  that might help you understand differences between MDF
https://mdfskirtingworld.co.uk/blog/the-difference-between-mdf/
https://www.weyerhaeuser.com/woodproducts/osb-panels/medium-density-fiberboard/mdf-product-specifica...
If you are interested in what Steve proposed I'd suggest sending a pm to him versus getting caught up with someone else's needless condescension 

I know that Mr. Vandersteen uses a special felt for marble bases under his speakers, you might want to call or email him.
Better yet, go to the new Vandersteen forum on the Vandy website.
Bob