My comment was that sealed "may also have noticeably higher distortion", and that this statement "can be debated". I didn't mean for people to take that as an absolute statement.
Oh, and I was also implicitly comparing sealed to horn-loaded in addition to ported. I know a few people here have suggested that bass horn sound has a certain "quality" about it that differentiates it from other types of subs. Why that may be the case is again a matter that could be debated.
I'd like to clarify for any lurkers that the discussion is specifically about harmonic distortion. There are various forms of distortion in a speakers reproduction though HD is often considered most important. Anyway...
Horns do seem to sound/feel different than direct radiators. Most people who've heard them tend to agree. The common wisdom seems to be that it's related to lower HD but I don't think it is, or at least that's not the whole ball of wax.