Dogmcd and Dave_b...All speakers, cone or panel, experience increasing distortion before their output rolls off. Indeed, distortion components tend to hold up the measured output level as the fundamental, which is what we care about, rolls off.
Which is why Soundstage won't even test speakers above 100 db SPL. (It would even damage many of them so they say - no matter that an un-amplified piano or drum set, trumpet, trombone etc. can easily exceed 100 db SPL)
A SW is a good way to protect speakers from damage and still get high SPL output as the bass is what requires the lion's share of excursion and power.