Just from my personal experience, A subdude or gramma do actually make a difference on hard floors (wood. tile, etc.). I live in a really nice house, but it was built in the 20s and has hard wood floors. My gramma pad eliminated a little bit of wood floor vibration from my floor where the wood floors had a little bit of play in them. Also, since my theater room is located upstairs and above my living room, I can from experience tell you that the gramma pad made the subwoofer much less noticable in the living room. Before the pad, the subwoofer was coupled to the floor via it's rubber feet and I could hear explosions, etc. much louder down there than I can now. Family in the living room usually don't even know I am upstairs watching movies or playing video games since getting the pad.
I also tend to believe that any kind of foam will do the same thing, even cheap packing foam. But for tower speakers that require stability, a gramma pad or great gramma pad cut in half securely gets the job done.