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?

I had a cabinet maker build a box (2x the foot print of the woofer) and filled it with sand. Plus I use BDR cones to keep it off the top for better isolation. Works for me!
You can place your subs on a platform supported by Vibrapods. At the cost no object level you can substitute Aurios Pros. Either will isolate far better than spikes.
Outside of working on the subwoofer situation, you should also take a look at how you have your TT's installed. Sean
The big concrete cinder blocks at the hardware store are $.90 each-you know the 8" by 8" by 16" at 25 lbs each with the two big holes in'em. Stack those around and slap a nice cover on the package--mainly for the TT.
2'X 2' X 2" Patio stones painted black look pretty good and do a good job especially with vibra pods between the stones and woofers. Glue a few pieces of felt to the bottoms to protect your floors or use 'pods on both sides of the stones.
If you have a crawl space below the floors by all means put "jacks" under the subs. Works extremely well.
Thanks for the feedback. The TT is in another room and that is a seperate problem. One thing at at time; I have to convince him his very worn B & O TT is a lousy turntable and my Systemdek IIx (for sale) is a much better table.

Purely selfless, of course...