Granite Bases for Vandersteen speakers

You're right about granite, it does all you say. Yours are custom cut versions of the machinists top plate that have been under my turntable and amp for many years.  https://systems.audiogon.com/systems/8367

Something I always suspected, but it took many years to hear it, granite imparts a hard edge. Always suspected, because hit it and it makes a high pitched ting or tink type sound. So its there, but might be hard to hear right now. It was for me. Meanwhile there's all the benefits you can hear and already described.  

Fortunately it turns out the mass makes a great base for springs, and the springs do a great job of erasing the hard edge ringing. I notice a lot of components still on factory feet. If you want to try a nice improvement, Nobsound springs on Amazon are very cost effective. Meantime, good work!

I wonder if IsoAcoustics footers would work even better, or perhaps those footers on granite would be ideal. I would think just using a granite plate would compensate for a bouncy floor, but at the same time easily reflect vibrations back into the speaker, which you would not want.

Adona equipment racks are made with granite shelves which are then bonded to an MDF layer with an elastomer layer in between. It addresses the ringing that you mention MC. I'm sure that this could be added to any granite slab. 

Constrained layer damping between layers of granite = yes. Note your Quattro cabinet with a cabinet is constructed this way already.
springs, springs, springs wrong answer for a hyper coherent, time aligned, phase correct Vandersteen. Pretty much every other speaker, yes. 

Tom - You and JohnnyR could test the theory w pair of VLR signature on a couple of the HRS isolation bases w springs.... 

have fun, experimenting:-)