Isolation for Granite Surface Plate

You can do a lot better! I've used different things on top of my 4" thick granite machinists plate and the best by far is Townshend Pods. They look good too, and adjust beautifully for leveling. https://www.audiogon.com/listings/lisa8492-townshend-audio-seismic-isolation-pods-set-of-4-sizes-a-b... Pods are sized according to the weight of your turntable. You can do the math or ask John Hannant at Townshend for his recommendation.  

The main problem with granite is ringing. This imparts a hard edge to the sound. You may not notice or may even hear it as detail. Until it is gone, and then you realize how much better and even more detailed the sound becomes. I went through all of this with mine. https://systems.audiogon.com/systems/8367 These are some old pictures I need to update them. Pods are now above the granite where you see BDR Cones and Round Things in the photo. BDR is very good but the Pods are quite a bit better. 

If you can't afford Pods, Nobsound springs from Amazon are awfully good for the money. (I used them before upgrading to Pods.) You will have to play around with them to get the right number of springs, and shim or something to get the table level. They are nowhere near the league of Pods but way better than you would think for how cheap they are. The main drawback with Nobsound is the lack of damping. You will eliminate the ringing edge of granite and gain quite a bit of imaging depth and detail. But if you go to Pods they do all that too and better, with greatly improved (deeper, tighter) bass, and instrumental tone and timbre so much better it is like getting a whole new upgraded turntable.

@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