What would you rather have come out of your steering wheel in a head-on collision? A hard marble slab, a hard metal spike, or an airbag?
What we are talking about is transfer of energy. Sound is just kinetic energy. Your speakers' cabinets vibrate, and those vibrations get transfered into whatever surface is touching them. The idea behind using a big, heavy, stiff, dense slab of something like marble or concrete is to try and create an inert platform for the speaker - something that will not vibrate. The problem is that it WILL vibrate. Maybe not very much - maybe not enough to be audible in any way. More mass takes more energy in order to get it to move, so having a very massive platform is a viable way to produce very little in the way of vibrations. The problem though is that - just like if a big marble slab came out of your steering wheel - ANY sort of energy that DOES get that slab to move gets transmitted VERY well.
So what about spikes? The notion here is that you are creating an extremely small surface area through which energy can be transferred. The big problem here is that there IS still surface area and the force of the energy does not change! Now, any and all of that vibration's energy just gets "focused" into those spike "feet" and you actually create a much more intense transfer of energy vs if you just rested the speaker flat, using its entire bottom surface area. Spikes are the opposite of reducing transmission of energy from the speakers into the surface below them. Instead, spikes intensify the transmission of energy! You'd actually be safer having a big slab of marble come out of your steering wheel vs having a hard, metal spike! At least the slab would provide a much larger surface area to absorb the force of the impact when your face hits it! The spike is just going to impale you! Much more intense transfer of energy!
Finally, we have the airbag. Instead of trying to remain immovable and inert or focusing all of the energy into a tiny point, an airbag deforms and converts most of the kinetic energy into heat. The force of the impact is spread out over the maximum surface area and rather than transmitting the energy, it transforms the energy and dissipates it.
THIS is what you want to do with your speakers. This is called decoupling. It is a "shock absorber" - a way to take any physical vibrations coming from the speaker cabinet that do exist and make sure that they never reach the hard surface that is holding them aloft.
You might never have a problem with your marble tops. They might be massive enough that the vibrations from your speaker cabinets are never forceful enough to get those marble slabs to move. The marble slabs would effectively be inert in such a case. But I wouldn't count on it. Hard materials like metal, wood, marble and concrete have their own resonant frequencies. At a resonant frequency, it takes very little force to get these hard materials to move A LOT because every tiny, little movement at that resonant frequency just feeds into the next tiny, little movement, and the next, and the next, and so on - eventually building a big, standing wave...just like that famous footage of a concrete bridge rolling like a giant wave when the wind got it moving at just the right (or in this case, wrong) resonant frequency. That marble slab might effectively be inert almost all of the time. But at a very few, very specific frequencies, it's going to move like crazy and "ring" because of resonance.
Spikes on the bottom of your speakers will only amplify that problem - making every movement of the speaker cabinets "felt" more intensely by the surface on which they are sitting.
The best solution is decoupling - the Auralex MoPads foam or some other "squishy" surface that will act as a shock absorber. With a decoupling device in place, your speaker cabinets can vibrate all they like. That energy will simply deform the decoupling pad and that deformation will transform that energy into heat and you will never get the problems of a hard surface beneath your speakers moving in sympathy with the vibrations of your speaker cabinets and creating additional sources of noise and distortion.
Embrace decoupling, folks! I don't know about you all, but I'd rather have the airbag every single time!
