Isolation for speakers on spikes

Generally speaking, soft items soften the sound, while hard items bring focus. I have used vibrapods, sorbothane pucks, brass cones, aurios bearings, and stillpoints.

The best item I have found is the stillpoint bearing. They give the benefit of spikes but maintain the speed and clairity that people tend to want.

Try Symposium platforms. Spikes are unnecessary with them.

Could be the board you are using in the configuration you described. I tried the same thing on my suspended wood floors which are carpeted. Vibrapods on the carpeted floor, MDF board on top of them, then the spiked speakers on top of them. Sounded OK but not ideal. When I substituted a 1" thick maple wood chopping block for the MDF the transformation was amazing: fast, clear, dynamic, musical and I've kept them that way since with a high degree of satisfaction; 2" would probably be even bettter. I got the chopping blocks from TJ Maxx for $10 apiece but you can also get them at Bed Bath 'N Beyond, Ross or a hardwood supplier for reasonable cost. If you can't find maple try other hardwoods. I've got bamboo cutting boards under my spiked monitors on stands and they, too, sound significantly better than the MDF or directly onto the metal stands.

Also, make certain you have the correct Vibrapods for the weight they are supporting.
Hope this helps.

Isolation will help prevent nuisance vibrations or resonances transfering from room to stand to speaker and vice-versa..."rattling" or "boominess" at certain frequencies.

Spikes make a very strong connection to a wooden floor: vibrations and resonances will transfer more easily. This can help if you have some useful vibrations somewhere that you want to amplify. For example, transfering low frequency enclosure vibration from enclosure to the floor for instance to enhance bass.

In theory damped systems have lower distortion overall - so my preference would be to use those vibrapods; Blu-Tak works great too!

All those vibrations in your speakers: where do all those deleterious resonances go with your Vibpods? Those 'pods keep the very microphonous meanies and the like right in your speakers, where you don't want them. You want them to go to Mother Earth through the points. This has been beat to death here on the 'gon. Some are couplers, some decouplers, and yes; some mix the two. Go figure. Why do you think the Sistrum Platform worked so beautifully? Pure coupling--magnifying the points capabilities with the us of the platform acting as a resonant conduit.