@baylinor : "Isolation is key, not coupling." As I understand it, the choice here has to do with the type of floor you have. If it's suspended wood, the floor becomes an uncontrolled resonator. It that case, you want to isolate the speakers' vibrations from entering the floor. If, however, you have a concrete slab floor, spikes are a better idea in order to keep the speaker enclosure from moving at all. Ideally, one wants the speaker cones only to move. But every action produces a reaction, so motion of the cones will be communicated to the cabinet, which then moves in the opposite direction, the result of which will be a slightly muddied sound on transients.
@fpomposo : I also have found sorbothane superior to GAIAs. I tried GAIAs, borrowed from a friend. Got some of the improvements others describe here, but so expensive. Then I found a company that sells various sizes and densities of sorbothane on eBay. Got round feet with threads that screw into the speaker's spike receptacles. All the improvements the GAIA made, and more: tighter and better defined bass, deeper and wider soundstage, better separation between instruments—in general, a clearer, more transparent presentation. Now, however, the tall speakers wobble a bit at the touch, as if they were on Townshend platforms.