The linearity of some monitors suffers with high levels because the voice coil moves outside the linear fluxivity region of the magnetic gap. But at low levels the voice coil stays close to the centre of the gap and so that's ot a problem...
Conversely, in ported monitors the loading on the bass driver changes with volume becuae of the acoustic impednace and the momentum of the moving aor mass inside the enclosure trying to fight its way out of the port(s).
As a result, ported speakers tend to go bass light as you reduce the listening volume, and this effect is emphasised by our own non-linear hearing response (as mentioned above).
Sealed cabinet speakers, electrostatics and ATL cabinets (PMCs) don't suffer this problem and retain a flat balance at low levels.
Hope that helps
Technical Editor, Sound On Sound