I recently experimented with spikes and other methods of isolating floorstanders. My goal was a little different, I was trying to remove bass vibration that I could feel at my sitting position. Spikes made no difference, neither did a buildup of carpet at the site (although one commenter suggested a thicker carpet over the entire floor - which I believe would have worked.) Anyway, I ended up sandwiching 1/4 inch sorbothane (the stuff used for running shoes) between thin sheets of plywood - essentially a single ply. Creating two layers. This caused a noticable reduction in the vibration. Never one to trust my senses when it comes to measurement (spent too many years as an engineer to know the flaw in that method of 'measurement') I created a measuring device out of a thin glass of water, placed on the floor. The deflection in the water movement up the side of the glass was less with the construction in place than without or with the spikes. This was on a wood floor in an old house. I cannot say that I noticed a difference in sound, but that was not my goal, and I noticed no difference in sound spiked, on the constructed sorbothane - plywood plates, or directly to the thinly carpeted floor. This obviously was at high spl levels.
What should be under my speakers?
I have Sonus Faber Grand Piano Home speakers in a 20' x 17' room with wood floors. The speakers have spike feet, and each one is sitting on a 1 inch thick, 12" x 12" slab of granite. Each piece of granite has a square piece of carpeting under it, as a further buffer. I know I probably could figure this out with a lot of experimentation, but what impact is the carpet likely having on sound quality? Would it be better (Subjective, I know) to have the granite slabs sitting directly on the wood floors? How would the sound change if I ditched BOTH the carpet and the granite? I would like to maximize bass impact, because these speakers are a bit thin in that regard. Thanks.