I've been using 16" bike innertubes with 1 1/4" marble slab on top. Had this under CD, pre, and amp on hardwood floor. I recently moved to new room with concrete slab under carpet. Now amp sounds better without innertubes, as does preamp. Rack is now spikes through to the slab. CD still likes it, allthough not as much (Rega Planet already has squishy feet, because its a top loader, and has no internal damping) I got the marble from a marble retailer's scrap heap, cut it with a masonry saw (lucky to have access). Some stores sell pre cut "Gardn Path" type stones, typically in maybe about an 18" square, sometimes slate, sometimes granite. Usually pretty inexpensive. IT is definately a good idea to put innertube under your shelf flat, and inflate it minimally, in my experience.
Air Bladder Isolation, Cheap
(As reported in Soundstage) I am very impressed with sonic improvements using 12-14" X 2" bicycle inner tube under CDP with mass loading on top. Local dept store bicycle dept has tube for $6 and hand air pump for $4. Using just enough air in tube to provide 1" height when fully loaded, and placing mass loading on top......you can buy Bright Star little rock $1-200, or buy 10-25lb metal barbell plate (can wrap in black dish cloth) or use lead shot in black sock etc. Provides deeper bass and more 3D soundstage than any cone or similar device, better than vibrapod, good for detailed SS systems, anyone else tried similar set-up, Sam