This is just my opinion, but I would stay away from Free air subs, and or Bass tubes. With free air, you still have to use some kind of board to separate the front of the cone to the magnet. Im just not a believer in this kind of design. Bass tubes, and or Bass links, are cheap alternatives to "real" subwoofers. Most good speaker manufacturers these days make pretty good "flat" subwoofers. These flat designs are not as good as regular woofers, but they allow you to build a box that is very slim and stealth. IMHO One of the better flat subs out there is the JL Audio 13TW5-3. This sub is real flat at only 2.5" thick. Remember when considering a flat subwoofer, that they still need quite a bit of air space to sound good. Its just that they are flat, so you can build a very slim box, to fit better in certain applications. The JL sub is not cheap at close to $600, but it is one of the better ones out there.