Slate under speakers?

My system - listening area is on a suspended floor loft. Wood floor with carpeting on it. As a trial/experiment I currently have formica covered countertop sink cutouts under each of my Snell Type AIII's. No spikes...just sitting on top of the carpet. The difference was an easily noticable tightening up of the sound...more I'm looking for a more permanent solution.

I came across some slate slabs (1 inch x 22 x 34 inches) that would would fit under the speakers nicely. Has anyone used slate in this way? If needed I could spike the platforms into the floor.
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.
Got mine on spikes, on top of 1.5 (or maybe 1.25, don't remeber) inch granite slabs (custom cut and polished by a stonemonger down the street), and the slabs each on a trio of those "magic" sliding furniture footers. I've never critically compared alternatives, but figured it's got to be better than sitting directly on a suspended wooden floor and I sure do like the way it looks.
Fishboat. Slate doesn't make sense. You would be better off with less density, surface area etc. Blindjims idea of plywood or even MDF would be much better on carpet. Wrap it with felt to make the wiff happy. The sound will just resonate back through your cabinets if you use slate. It doesn't work.

Slate doesn't make sense... The sound will just resonate back through your cabinets if you use slate. It doesn't work.

What is your basis for these statements? Do you have first hand experience or are you just theorizing or imagining what might happen?

In fact, there are sound (pun) reasons for isolation of this type for suspended floors and quite a few AGONers have reported success with this method. It worked extremely well for me and the results were measurable, so I'm concerned your absolute statements might misslead others from trying it.