Floor standing speaker isolation advice needed.

Stiff Springs under a wide footprint board supporting the speakers should do the trick. It’s two, two mints in one! Isolates the speakers and isolates everything else from the speakers. 👯‍♀️ Mass-on-spring. Accept no substitutes.

Within the budget you mention, I think you should consider @geoffkait's suggestion.  He does understand the mechanics of springs. I am personally familiar with the Townshend speaker podiums which perhaps are above your budget.  The Townshends are superb at the kind of isolation Geoff is talking about.  But I expect Geoff's solution will get you much of the way to the Townshends' level. Springs in my experience are better than other devices for isolation.  I have a bunch of springs (Geoff's actually) under my other components though not my speakers. 

I would nix the granite too.

If you want great  improvement get Symposium Super Plus bases with copper spikes.Im using them under my Magico Q3s the improvement in bass definition and detail was staggering.

Whatever form of isolation you chose (sorbothane or springs) will need to be compliant/soft enough to get as low as possible (<10Hz).

That will make the speakers very wobbly so will you need to use some form of wedges underneath the speakers or the platform with a minimum clearance of say 1-2mm to prevent accidents.

The benefits of isolating speakers have been well documented. Spikes induce resonance in the baffle hundreds of times greater than that of even soft rubber when the speaker is in use.

The problem with isolation is that it’s cheap to implement and the sonic gains are huge (bass starts to play notes) but apart from a few companies like Townshend or Herbie’s there is little profit in it. Whereas with spikes the opposite is true.