I'd like to share my thoughts regarding spikes; I performed the investigations way-back which led to our incorporation of spikes in the CS3 in 1983. As Sandy said, the chief problem is recoil. The effects, however, depend greatly on the floor system and are generally more evident in odd ways.
On carpeted floors the speakers sway and those motion effects become quite significant at high frequencies. A firm carpet may exhibit little to no symptoms, but a foam pad or bouncy carpet may allow considerable movement. On the other hand, coupling to a bouncy / resonant floor may introduce greater problems than allowing the speaker to float on the carpet. Some floor systems can be stimulated into resonant modes by spike-coupled speakers. Those vibrations can often be felt via bare feet in the listening position, offering clues to what's going on.
The sonic effects seem to congregate around image stability. The subtle spatial cues that convey image specificity can be scrambled by a moving speaker. Vague imaging, especially front-to-back depth, can be caused by unstable speakers.
Early Thiel spikes , (up to at least 3.6) were 3-point, non adjustable - explicitly defining a plane of contact. Those spikes had 3 lengths for various tilt strategies to arrive at 3' launch point aimed at your seated ears. Later models adopted 4-corner adjustable spikes. Caution: if those 4 spikes are not very carefully adjusted, problems could result from the insecure foundation.
This afternoon I tested my 2.2s under development. My floor is glued 5/8 + 5/8 plywood on 2x12 joists on 16" centers. That's stiffer than many domestic floors. My covering is commercial (old ski-lodge) tight, hard rubber backed carpet squares. My spiked speakers transmitted a little more vibration to my bare feet. I sum to mono in my preamp and pan left or right speaker for comparison using self-recorded material - in this case Dana Cunningham's Dancing at the Gate - with lots of detail. The spiked speaker produced more subtlety, nuance, detail, complexity. Highest single notes sounded more dimensional and more ambience was apparent across the range, with bass decay remaining more musical for longer times.
The improvements could be reliably noted in my setup. These are the kinds of performance particulars which I hope to increase with upgrades. However, I must note that this incisive precision is not always appreciated. Many manufacturers purposely make cap bundles to spread out such transient information; and I suspect that many listeners would find the added detail to be a negative, especially with recordings that lack the spatial and ambient clues that add enjoyment when present.
To batmanfan's original question: outriggers will do more than plain spikes. And I don't know the configuration of the bottoms of his speakers. If they are flat like the 1.6s, then some kind of feet are almost certainly an advantage. (I have seen marbles set into the corner sockets.) Threaded feet allow more precise bearing. And if your floor is resonant, then some kind of isolator pad might help decouple from those resonances.
Have fun.Tom
