Hi Marc,
Optimally, a person should use a platform that has as little sonic character of its as possible AND that platform should absorb as much of the vibration and resonance out of the component as possible. Preferably, the platform would efficiently convert the mechanical energy (vibration) to a more benign form of energy (such as thermal energy - heat).
In addition, the support for the component should provide an effective barrier to stop vibration from entering the component from the floor/rack/shelf. In addition, there needs some mechanism to minimize the effects of air-borne vibration that is striking the component's chassis directly from the speaker AND addresses internally generated vibration within the component (motors, humming transformers and cooling fans).
Please be aware that natural wood, plexiglass, acrylic and many other similar materials are resonant and stone (granite, marble, corian, cement, concrete, glass, tile and other very rigid materials - IE: metal) will ring. The ringing and resonance will be transferred into the component and will negatively alter the signal flowing through the component.
Some people are confused about the fact that even though natural wood may be the right choice for a musical instrument (because it has distinct resonances) it is not appropriate for vibration control because of that very same reason. The components which comprise audio systems ARE NOT musical instruments. They should not have their own personalities (colorations) or have resonances imparted upon them by inappropriate choices in vibration control materials. The components in our audio systems are used to reproduce the sound of the original musical instrument as it has been captured in the recording. Anything that alters the signal flowing through our system's components takes us further away from being able to faithfully reproduce the signal in the recording.
Best Regards,
Barry Kohan
Disclaimer: I am a manufacturer of vibration control products.