I have another direction for you. Spikes do provide some isolation for your speakers from the floor.
But spikes only allow the resonance of the speaker and the floor to interact less with each other.
The benefits of course coming from 1.) raising the speaker higher off the floor. 2.) decoupling the speaker from the floor. 3.) lowering the amount of vibration sent back into your audio system via the floor.
Try this, I think you'll find the results more musically satisfying. These work very well on carpeted floors.
A) Place you're speakers on Vibrapods (4 of them). You can either cut a board the size of your speaker to place on the carpet. Then put the Vibrapods between your board and the speaker.
What works better, but costs more would be to place Black Diamond Racing "things" under the Vibrapods rather than using a board. This is also the more attractive approach.
B) An even sonically better approach is to try some Stillpoints (point up) between the speaker and the carpet.
The Stillpoints will absorb speaker vibration, where spikes or cones will just change the resonant frequency reflecting some back to the speaker.
Stillpoints are the best I've tried. They seem to be very neutral and don't add an additional character as do brass, vinyl or carbon fiber. This also isolates the speakers (via the floor) from the components much more. Most of the vibration that makes it's way into your components is caused by your speakers. Either through the floor to the component rack, via speaker cables, or via the air.
These solutions address only part of the vibration feedback loop between component and speakers. To really inhibit this loop, resonance control needs to be addressed at the component level also.
You'll be very pleased with the results. Now if I can come up with better ways to deal with the resonance via the speaker cables. That's another topic.
If you'd like to try out either of these solutions, I can loan you some of these devices. Here is some Stillpoints info: http://audiobuff.zoovy.com/product/Stillpoints .