Isolation feet for monitors

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.

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

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

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?