If the speakers are extremely heavy that is mass, which is in the kilograms, while the moving parts are cones and voice coils, are in the grams. What this means, if you know your physics, f=ma force equals mass times acceleration, you can have a hell of a lot of driver cone acceleration while moving the speaker mass only microscopically.
Still, this microscopic movement matters. Because the finest details we hear are so low in level they correspond to cone movement measured in angstroms. Really, really small.
Which might argue for rigid mounting. Except it turns out the bigger problem is once the mass does move it transmits this vibration down through the cabinet into the floor and from then on the whole speaker/room system is vibrating.
This ringing goes on a lot longer than if the same vibrations were confined to the speaker cabinet alone. It is the duration of the ringing and not the amplitude that blurs and smears and loses detail and dynamics. This is visually demonstrated by putting a seismograph (in the form of an iPad) on a speaker. The speaker on spikes shows obvious prolonged ringing. The same speaker on Podiums shows virtually no ringing. The sound we hear corresponds perfectly to this demonstration.
There is indeed some loss of dynamics. But there is also the same loss even when supposedly rigidly mounted on spikes or cones or whatever, even on concrete. This happens because of another physics feature, leverage. Drivers are always some distance from the floor. This distance is a lever arm. Anyone can prove this, simply push on the top of your speaker, believe me it will move. Push hard enough it will fall over, spikes or not. But even a small push of a fraction of a pound will with tall speakers leverage into lots of pounds at the base.
The speaker rocks. Either way. Virtually the same amount. Main difference, springs make it easier to see. Real difference, springs allow the vibrations to dissipate much faster. That is why they sound so much cleaner, in spite of superficial appearances.
