Isolation feet for monitors

Try Auralex MoPads (www.auralex.com).
These are foamblocks made from the same acoustic
(high mass) foam as the soundproofing in recording studios designed for exactly your purpose. But most of all they just work! No more rattles and the bass has gained more definition since I put them under my Little Reds despite them pushing the MoPads weight limit.
Your approach of high mass and decoupling is definetely the right way of dealing with your problem,only ever use spikes on concrete floors or your shelf or wooden floor effectively becomes part of your speakers cab! And who wants to hear the sound quality of their shelves or floors?

If you want to isolate your monitors, rather than couple, you will need to do something similar to what I suggest; Vibrapods or sorbothane...Chris

I am pleased with what I hear when I use Audiopoints, which are made from brass and made by Star Sound. The Goldmund cones are also very nice however much more expensive

Timken bearings with small DH Cones

You might not need the DH Cones with your granite. This setup will cost you $80 to $120, depending on the cones, but would probably sound incredible.

The isolation is to stop the speakers and book shelves resonating together - the shelves are very light and thin so i sometimes hear a tze, tze at high volumnes. So perhaps weather stripping between the speakers and granite bases will help to bind the speakers to a massive base and then some neoprene between the granite and shelves will help to stop the shelves resonating at higher volumnes. I will give it a try

I'm curious why you feel the need for isolation. Any type of flexible feet will allow the monitors to move about. My experience with monitors is that the more rigidly they are held in place the better they sound.

Wearherstripping is what I have used under my monitors on a number of different platforms. It's real cheap too. It comes in different densities and thicknesses. It's worth a try.

I have Soliloquy 5.0 monitors and I've tried many things underneath them. The best by far are #2 Vibrapods. Enhanses the deeper bass and its definition and tunefulness. Clarity and microdynamics are also excellent.

I also achieved very good sound with Small Isonodes by Bright Star Audio. The ultimate in clarity, but they did suck up some of the microdynamics. Fleshes out beautifully though when turned up.

Mine are almost 4' from the back wall however. Yours look to be pretty close.

Chris