If you are really curious about stuff like this, go over to avsforum.com and search the DIY speakers & subs forum for user Mark Seaton. He has made a number of excellent posts where he talks about sub desing and tradeoffs.
He has a small company that makes Seaton Submersive subs that he designed right. Everybody talks about them and how great they are, but you can't find much info, like pictures, frequency response, etc. I know that they are supposed to have dual opposing 15" drivers in a box and that's about it.