Lew,
To give a bit of background, I'll take you back several years. My first e-mail to Jean expressed an interest in building a turntable from cement or machine grout. I abandoned that project shortly after, but the idea always stuck in my mind. Later, when slate hit the scene, I remembered the idea, and looked for similarities. There were many, and working with slate appeared at first glance to be easier. Shortly after that, I started my current project which makes heavy use of slate.
What I have learned, so far:
1) If you combine plinth materials, (I haven't) be sure to study which one should go where. I say this because slate is astonishingly effective, and it should be used in those areas that generate the most noise and vibration.
2) Building with slate isn't an easy road to travel. If your project is complex, there are so many issues to be aware of that it boggles the mind. Is it flat enough? What glue should be used? The list goes on. Also, using slate is a lot harder and slower to work than wood or acrylic. Still, it can be done with normal tools, unless you require extremely precise cutouts in your design.
3) Most mistakes are expensive ones. A small error can quickly take you past the point of no return, so be very methodical in your approach.
4) Slate is the most effective constrained layer material that I have ever used. I mounted a motor directly on the surface, and could not hear it with a Litman pediatric stethoscope from less than a centimeter away. It is very impressive in that regard, but not dead like lead.
5) Soft slate works great. If you happen to lightly chip it, it is easier to fix than some harder material.
6) It's pretty, too. ;)
Regards,
mosin