Hi Chuck,
It is important to keep in mind that vibration that affects turntables (and all other components) does not only come up through the rack from the floor. The most effective vibration control product will not only provide a barier to floor-borne vibration but will also have the ability to absorb and dissipate unwanted vibration out of the turntable's chassis that has been created inside the turntable by the spinning motor and platter and has traveled directly through the air from the speakers towards the turntable's outer chassis. Most vibration control products only have the ability to deal with floor-borne vibration and the majority of those are limited in their ability to do even that effectively.
It is also critical that the vibration control product not add its own contribution to the signal flowing through the comoponent. Vibration control products that are constructed from materials that ring (stone, metal, glass, etc.) or materials that resonate (wood, acrylic, plexiglas, plastic, etc.) should be avoided.
Best Regards,
Barry Kohan
Disclaimer: I am a manufacturer of vibration control products.