Do we even know what excursions were taking place when this distortion was taking place?
The HT Shack Subwoofer test graphs are colored with a variety of measurements at various SPL outputs, all at less than insane levels. Many of the curves show what could hardly be called insignificant amounts of distortion.
If you look at an equal loudness contor plot of our hearing sensitivity then you will observe that the sound of 10% distortion will be much louder to our ears than an orignal 20Hz tone => because of our lack of hearing sensitivity at 20 Hz you hear the distortion louder than you hear the music.
Refer to the bottom of the page (last two plots) to see what
Seigfried Linkwitz has to say about distortion at low frequencies.
Linkwitz suggests that 1% THD is the kind of distortion you ideally need in a sub - if you want to hear what is on the recording.