There are a lot of strange things out there in the world of subwoofers... various materials, some made of poly, some of metal.  There are even square subwoofers.  There are enclosures with 3, 4 or more woofers... and some with only one.

Which of these "unusual" or lesser-known features of subwoofers really deliver the goods?  Have you found these features to benefit the sound of the woofers, or are they just voodoo? What's the coolest "special" design you can think of?