Yes sir you are correct,for my friend Jocelyn 24hz is not low enough, with the addition of two JL F-112 subs his speakers go down to 14hz that is his choice, if it was up to me there would not be any subs but it is not my system it is Jocelyn's system, live and let live.
PS: I don't like Subwoofers with panels // boxes with boxes and panels with panels, here is one sub that i wouldn't mind.