DIY speaker isolation base for a wood floor

I concur granite is an excellent material, no doubt due to its inherent stiffness and mass (inertia). I have been using granite with my springs for isolation for more years than I care to remember, assuming I could remember. Ha ha Another excellent material is a slab of Bluestone, I.e., paving stone, a similar but less expensive choice. Available at many but not all Home Depot. The mass and stiffness of these slabs are just what the doctor ordered to push back against the bending forces 🔄 of seismic waves. The springs are rather good at dealing with vertical forces. 🔝

Ok Geoff I'm trying your idea of springs and a granite slab or 2. I would assume 2 stacked would be better. I'm going with nothing in between. I'm picking up the springs Monday. I will tinker with that. I will then see if the sorbethane between the slabs is a bust with the springs. Maybe skip that and go with the spikes. Thanks for your expertise here and thank everyone else for there sometimes passionate opinions.

The granite slabs should be fairly large in dimension, maybe 18x18. 24x24 would be even better, more stable. If you can find one slab two inches thick, use that but only one slab. You want to reduce Center of Gravity height, not increase it. How heavy and tall are the speakers?

Use as many springs as you need to get the whole thing stable. Minimum 4. Might need more than 4. Did you calculate the spring rate of the springs based on total mass of speaker + Granite?

Thanks Geoff here is what I know.
102 lb load rated springs.
Spring constant 126.25 lbs per sq inch.
1.375" OD x 0.177" steel wire and 2.75 " length.
The slabs weigh 32 lbs and I  have four
The speakers are around 72 lbs.
I can get more when I go to town. I can't wait to try these. Thanks

It sounds to me like your springs are more suitable for a 400 LB speaker. The spring rate of the system is the spring rate of a spring x the number of springs.