I once read a test of the Infrasub 18 that claimed the harmonic distortion at the low end was as much as 10%. I can't for the life of me remember where I saw it. :-( Compared to servo-controlled subs that can be < 1%, it's a pretty big difference. However, it's not clear to me that most people find some amount of distortion on very low end sounds subjectively unpleasant.

I've only heard Bag End gear in professional (i.e. club) settings, and I generally thought the base sounded pretty good.