I had this assumption also until I read "mynym's" comparison of a 15" Tuba HT compared to a 18" LLT. He states the Tuba wins hands down.I've yet to see a horn subwoofer design that offers a better balance of performance than a good LLT dollar for dollar.
http://www.avsforum.com/avs-vb/showthread.php?t=1194093