I would recommend this one from JL audio:
I would have gone with the 12" version, but under my gaucho there was just enough room to house the 10" one and still have all of the other stuff that I normally store under there as well. I purchased the optional level adjusting knob, and that works well in situations like DaveB mentioned. If I'm watching a movie/show I can turn the bass up or down as needed. The 10" puts out way more bass than I need, so I can only imagine that the 12" is pretty awesome. Even with the 10", my low bass extension really impresses me. I used to have two JL 12's in a ported enclosure in my nissan hardbody, and that had lower extension, obviously, but these microsubs are pretty damn impressive. If you get the chance to check one out at a store it might help. They are super easy to wire up and having the built-in amp is a real plus. Also, the wiring plug can be pulled out of the sub to allow you to remove it easily, without having to unhook any wires. This is a good thing to have if you absolutely needed the extra space.