A lot of good points were made in that thread, it was a pretty good discussion. Josh Ricci also had some interesting comments about distortion about a week ago here
. This is why I say that the component distortion charts in subwoofer tests are more important than THD tests. Second Order Harmonic Distortion can comprise a large part of THD tests, but it can hardly even be heard unless it is really bad (even then, it will probably be masked by other sounds being played). A lot of people even prefer the sound of some harmonic distortion. Some subs are mistakenly perceived as more articulate because they are mistakenly producing higher frequency sounds, and since higher frequency sounds are more easily discerned, they are erroneously thought to have more definition.
Everything being equal, I would rather my subs have no distortion, however if I see a moderate amount of lower order harmonic distortion in subwoofer tests, I do not regard that as a big deal.