I'm not good at physics, so feel free to tear down this hypothesis (only for speakers on stands).
For every action there is an opposite and equal reaction. So, when the speaker cone moves back and forth, some of its energy may get spent moving the speaker cabinet in addition to moving air.
Setting the speaker cabinet directly on a heavy stand couples it to the stand and thereby provides more mass to resist movement of the speaker cabinet. Spikes would seem to de-couple the cabinet from the heavy stand, thereby defeating the mass advantage of the heavy stand. So, spikes allow some energy that could go into moving air to go into moving the cabinet.
Also, sitting the speaker on spikes seems to encourage more movement of the speaker cabinet versus the speaker cone by reducing the drag or friction between the speaker and its stand.
It seems like one would want the speaker coupled to a heavy stand (either in direct contact or, if the stand top is slick, on something with a lot of friction such as rubber) and the stand on spikes. OK, fire away. Jeff