I've just removed the spikes from my Monitor Audio S8i floorstanders and they're now sitting on some pipe insulation cut into 25mm thick discs which sits on a slab of marble about 25mm thick.
The difference in the sound is quite amazing, the MA's have always sounded a little bright / hard and bit thin in the mid range but now they are a lot smoother at the top end and the mid range has filled out quite nicely. I do seem to have lost a bit of tightness in the bottom end though but nothing major. For the money, this is one of the best upgrades/tweaks I have ever made.
Get down to your local DIY store and get the biggest diameter (about 65mm) pipe insulation you can get, cost: about £2.50 or less. Get some granite/marble chopping boards from your local supermarket Cost: about £10-£12.
Ian