An explanation of stage or floor coupling
Part 1 - Mechanical coupling
The key point to understand regarding coupling is that there are two forms of coupling - mechanical coupling and acoustic coupling.
With mechanical coupling the stage/floor is excited by the vibrations from the cabinet or by the acoustic output from the speakers/port and the floor itself becomes a resonating surface like a giant drum skin or speaker cone. The acoustic output from the resonating floor is very unpredictable, centred around the modes of resonance of the floor, which almost always results in a boomy onstage sound and unpredictable offstage sound (depending on how it reinforces and cancels the output from the speaker cabinet).
You should generally try to avoid mechanical coupling and this can be done by using a well-braced speaker cabinet which vibrates less, isolating the speaker cabinet by reducing the amount of physical contact with the floor (isolating foam/tilting cab/raising onto beer crate) and placing the cab at a more solid part of the stage (i.e. a corner). If you are on a small hollow stage see if it's possible to place the cab off the stage - also make sure that the PA subs are not on the stage, though any sound engineer with a clue will not have done this.
Part 2 - Acoustic coupling
The second form of coupling is acoustic coupling and this works by the direct output and the reflected output from the speaker cabinet combining to give increased output. As you can read in the understanding sound dispersion section the low frequency output from any loudspeaker is omnidirectional which means that your bass cab is sending low frequencies not just to the front but also out to both sides, upwards, backwards and downwards! The best way to picture this is as a sphere of energy surrounding the cabinet with the cabinet in the middle of the sphere.
If you place your cab on the floor then the bottom half of the sphere is blocked off and that energy is reflected by the floor, combining with the upper half of the sphere and increasing your low frequency output by ~3dB - that's equivalent to going from a 100W to a 200W amplifier. This kind of coupling is thus a really good thing!
So what happens if you put your cab on a beer crate so you can hear it better and/or to stop some boomy sounding mechanical coupling with the stage - does that mean you lose all of that 6dB of acoustic LF coupling? Fortunately the answer is no - what happens is you still get the 3dB gain but the cut-off frequency goes down, so rather than you getting the gain below about 250Hz you might only get it below 100Hz.