After much experimentation, I ended up with a similar solution for my previous room. The speakers were spiked onto slate for stability, which was then resting on carpet/pad which provides decoupling. Decoupling the speakers from the suspended floor reduces the ability of the speakers to drive the floor's resonant frequency. Do not spike the slate to the subfloor, as it will negate the decoupling.
Incidently, I kept the same slate in place over carpet in my current room despite the fact that the floor is concrete. It still provides the best and tightest bass.
Incidently, I kept the same slate in place over carpet in my current room despite the fact that the floor is concrete. It still provides the best and tightest bass.