I've got spikes on my M80's and EP500 and they go all the way through to the concrete. We just cleaned out carpet in the den and there was little evidence that anything had been there except a pencil width hole - convenient for putting them back exactly were they came from. I had to turn them upside down, though, when I moved them to a room with hardwood flooring.
By comparison, the couch and rockers have rubber feet and left a very large impression that didn't come up.
Scott My HT