Speaker shock absorbers

I have had great success with "Unhappy Balls". They work in this application to 1) absorb vibration 2) do so while remaining stiff. Halve the large size (1-1/2" Diameter) ones for a secure fit. http://scientificsonline.com/product.asp_Q_pn_E_3039129

I do not necessarily want to eliminate that effect. My only goal is to get rid of any transmission that is not airborne so to speak.

Heavy underlay with heavy carpet on top is by far your best bet. Try to dampen the entire surface rather than just the interface beneath the speakers.

If you have to go upscale, as I did, you might like to know that Herbie's Grounding Bases worked very well under my spiked floorstanders... at a price.

Why aren't you using spikes? Do you imagine most speaker manufacturers include them for decoration?
Spikes actually 'decouple' the speakers from the low-frequency structure-bourne feedback able to be carried in most timber-framed floors. These large-wavelength low frequencies are unable to 'cross-over' the tiny contact points of the spikes. Any rubber or foam you introduce between the floor and the speakers will simply ' bounce' with the feed-back induced movement of the floor and if anyone out there considers 'moving' speakers to be a good thing, they may not be the best people to listen to?

Halcro - I am well aware of spikes and have used them - that was not my question - your answer is non-responsive to my question and, hence both irrelevant and of without value to me. My question was simply where people who had used the method that I described found their materials. If I cared about what someone thought of my proposed method I would have asked a different question.