I have just purchased a MTX Lolita Low-Profile Subwoofer. My current sub is a beast and was showing signs of failure (had it for about 10 years).
The Lolita from the reviews that I have read is a pretty decent sub for the design limitations of being a low-profile design. My reasoning behind getting a low-profile sub is because I am slowly trying to get rid of my big bulkly speakers for a more modern look. I have not heard this subwoofer yet, I went strictly off the reviews that I have read.
The subwoofer will be delivered (hopefully) sometime later this week. Once I get is all setup, I will post my thoughts on it.
*1/21/2009 Update*
Had about a week of testing this subwoofer and I am very happy with the purchase. It is tucked behind my TV stand (only had to move it out about 2 inches to accomidate the sub. It would have been nice if it fit below/under my TV stand, the stand is just a bit too low for that. Listed to some music with heavy bass and the sounds was clear. It does not output the same amount of bass as my previous one (dual 8in), but I was not looking for a direct replacement. I was was looking for a sub that could easily be hidden.
Very satified with the sub. Next is to play some movies and see how it performs there.