Somewhere on Richard Vandersteen’s site there’s a paper on the subject which echoes my own thoughts.
You want to minimize the contact area between the speaker and floor. You also want to secure the speaker from rocking against the force of the woofers. The motors are powerful, and the farther from the ground the easier time they’ll have moving the speaker back and forth and causing Doppler distortion.
This is why sometimes something as simple as adding significant weight to the top of a monitor will tighten up the bass.