If you scroll down on the following link, you'll see an older NRC test of a single M2ti back in 2002 done by Soundstage.com of THD at a high input level --- 90 dB SPL at two meters (a little more than 6 feet away). That's approaching the "very loud" level, and the rise in THD shows you the effect of a small woofer "working hard", unaided by a subwoofer.http://www.soundstagenetwork.com/measurements/axiom_m2i/
Distortion is still under control and might be audible as a "fat" kind of mushiness or coloration in the bass region below 500 Hz. If you added a subwoofer and repeated the measurement, the THD would be reduced but not totally eliminated.
In past NRC tests of speakers for the magazines I edited in Canada, the test would be repeated at 95 dB SPL (that's very loud subjectively) and if distortion was excessive, we'd discontinue the measurement. That was the case for most compact 2-way bookshelf speakers.
So the M2 still has lots of headroom for loud playback of musical peaks without excessive distortion.