Thanks folks. But my question was more theoretical than practical: as far as practice goes, I’m sure my system sounds better with the speakers decoupled on Sorbothane feet. But the theoretical question remains: WHY? It would seem, as my "expert" friend argues, that a rigid structure for the drivers would be better. It makes intuitive sense that the motion of the speaker cones will necessarily move the entire speaker enclosure on that "floating" platform, even if in only very tiny amounts. This—again, it would seem—should "smear" transients, especially at lower frequencies, which should result in a subjective experience of reduced clarity, locational specificity, and so forth.
Only Eric has addressed this, and he only very briefly. That having woofers closer to the floor would be better is consistent with my intuition here: the moment of inertia on a tall tower (49"), if the woofer(s) are mounted high, will be greater, and thus the movement imparted to the enclosure will be greater. As it happens, my speakers employ the "D’Appolito" array: two 6.5" woofers, one above and one below a dome tweeter, all three of them at the top of the tower. And yet, the speakers DO NOT sound smeared or compromised when standing on rubber. Why not?