If it's producing "clearly audible" sound at 17hz, then I'd venture to guess that THD is rather high.
20hz is really a lower limit, not an average, to human hearing. In fact, most people can barely hear below 25 (give or take) and what you hear is mostly harmonics (same is true with 20khz, although there are some of us unfortunate souls who can hear up a bit past 20KHz) from our relatively (to the mains) inaccurate subs.
But I'm sure I'm preaching to the choir here...
EDIT: OH look! While I was typing, we got the specs... 3% at 90db would definitely be "clearly audible", especially at 110db where it's probably closer to 12% or even higher. I'd cross this sucker over lower than 30Hz if I intended to use it.