Indeed, having read something similar in an old audio basics, I went to my local stonecutter and had him cut two 3" slabs of limestone with the exact footprint of my subwoofer. One goes underneath to help decouple from the floor, and the other goes on top - and it's probably 40-50 pounds. The stone was cheap, it added slightly better clarity and definition, it looks great, and keeps the top of the sub from getting marked up when I use it as an end table.
My stonecutter thought I was nuts, but he didn't complain when I asked him to cut a rack for my entire system plus some stands for my biro L/1s.
Can you post some pictures of that, just so I can see something I have never seen before. Espically interested in the rack, that sounds kind of neat.