A bit unfair the way this one is bucketed - I ended up choosing extension.
But putting down maximum extension is a bit misleading, though.
First priority:
A sub must be able to reproduce down to 20 Hz at reasonable SPL levels - reference is nice, but as a married guy with kids I don't do a lot of listening at those levels. Too many people don't know what they are missing in home theater with subs that only extend to 30 Hz. Or worse those HTIB subs that are no bigger than some bookshelves I've seen that extend only to 40 Hz.
Second priority:
Quality and accuracy - not sure what these mean, but the sub should not distort, rattle, or resonate at reasonable listening levels.
Max SPLs ranks pretty dang low for me. I don't listen at reference levels. I have a wife and kids at my opportunity to do so is very limited.
Aesthetics somes in last. Lets face it, most if all subs are not pretty in the common sense of the word. They are not furniture either - you shouldn't set stuff on em. I'd rather have a black sub that covers extends down to 20 Hz and below with minimal distortion, resonance, than a beautiful sub that my wife puts plants on and doesn't get the job done.
~Josh