why spikes under speakers???

It's all about reducing transmission of cabinet resonance from the speaker to the floor. This way you hear the vibrations of the driver, not the vibrations of your floor. Energy transferred from the speaker cabinet to the floor effectively turns the floor into one large transducer and, as you can imagine, a floor doesn't make a very good speaker driver. If a perfect speaker cabinet with no resonance could be designed, then there would be absolutely no benefit to putting that speaker on spikes. Unfortunately, no such perfect speaker cabinet exists, so we put spikes under our speakers to cut down on the transfer of vibrational energy to the floor. Incidentally, this is de-coupling...not coupling. Spikes do not, nor ever have coupled a speaker to the floor in an acoustic sense. Yes, they do anchor a speaker in the physical sense, but that has nothing to do with acoustic coupling. This will start a war, but I'm tired of hearing it wrong. Acoustic coupling refers to efforts to maximize vibrational energy transfer. To do it with a speaker, you would remove the spikes and put weights on top. Try it and see how that sounds. Then try de-coupling with spikes...you'll never want to be without them again.

Actually, woofers can move enclosures. Try putting your cell phone on vibrate mode and set it on the counter. Watch what happens when it rings. It doesn't stand still..

thanks guys!

I have achieved substantial improvement in the sound of tower speakers that were already spiked on a carpet, by placing them on a spiked stand on the carpet. I think that having the speaker lifted away from the carpet (even that it was already on spikes) seems to help the sound. I can't even begin to speculate why this is true. I don't think its tweeter height, as I do experiment with seating position height. I've done this with with several pairs of Thiels, Proac 2.5, Spendor FL-9 and now Silverline Sonatina II's. The degree of improvement always surprises me (you'd think I'd learn, but nah... ; - )
Art

Just to add to Goinbroke's correct explanation of what spikes are all about, the decoupling of the speakers will give an impression of less bass but in reality there will be more extesion in the lower region (less boom) and not only that but the overall sound will be cleaner.