Subwoofer isolation

A friend and I both live in old homes with hardwood floors. Needsless to say, when we crank it up our various subwoofers cause problems for the turntable via vibrations that turn the floor into an extension of the woofers.

I was thinking a slab of granite/marble/dense fibreboard made to place under said woofer would be a good thing.

Any suggestions on materials, construction, and so on?

Regards,
Carl