Slate under speakers?

Yes. I have my Dynaudio speakers on 1" thick granite slabs.Tightened up the base

Slate is becoming a favored material for custom turntable plinths due to its characteristic of dissipating micro-vibrations. It is not simply mass as several other materials can offer that as well. Apparently it is the layering process when the slate is naturally created.

My guess is that slate will become more popular for a base under component as more hobbyists learn about it. I would think it could be a superior material under speakers too.

I use Adona 24" X 24" granite/mdf platforms under my speakers & the results are amazing.
Better base is just the beginning as I was able to place the speakers closer to both the rear & side walls.
I must disagree with the use of spikes - I can now position my speakers all by myself - which is a major accomplishment (especially on carpet) as the speakers have 4 spikes & discs attatced to the bottom of the metal bases.
My new listening room has cork floors over concrete & the speakers are further from the rear walls but still very close to the side walls & the sound is still fantastic.
Probably the best $500 I have spent on any tweak!
BTW - The graphite SF Stradavari's look even better on the granite bases which is another plus.

I use the Adona platforms as well for my speakers, granite side face down with speakers sitting on the MDF layer. Have been very pleased with this arrangement.

This is a very interesting thread as I spent last weekend doing a similar project. I too was trying to raise my speakers up a bit to suit there location. I took 1-1/8th MDF I had left over from when I built my house, I spray painted it black and installed adjustable felt feet that I purchased at Home Depot. The felt is very dense and I can now move the speakers by myself which are heavy. My speakers have spiked feet and are sitting directly on the MDF platforms. My floor is porceline tile over concrete slab so it's a very hard surface that the felt feet sit on. To my surprise the base is at least as tight as it was and the sound did improve for me by raising up the speakers.