some of the really good high end HT subs are closed designs, such as Jl audio's gotham sub.
http://home.jlaudio.com/products_subs_pages.php?page_id=3

I got to hear this sub at a local HT store and watched some clips from spiderman 3 on bluray, as well as others.

Let me tell you, that thing sounded AWESOME. IT was actually the first non-ported HT sub I have heard, and was def. impressed.

-David