Isolation for Granite Surface Plate

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

oldskoolmark

Try a "half inflated" kids scooter innertube under them, complete (air) isolation worked wonders for me back in the T/T days and now with CD transport also.
Pick your size. (Tom Gillett aka: Sam Tellig of Stereophile put me on to this decades ago)
https://www.walmart.com/search/?query=scooter%20tube&typeahead=Scooter%20tube%20

Cheers George