I suggest the Velodyne in-wall subwoofer. It's what I'm using right now and I'm very pleased. However, when designing my system, I wanted everything embedded in the walls; including the tv and other devices. You may not want a similar design. Therefore, I suggest building your own sub as they are easy to build, cheaper, and of the same quality. Furthermore, I suggest incorporating the sub into the sofa. I haven't done such a task myself, but my best friend has and believe it or not, the results were great.
EDIT: Scratch the sofa idea if this is your first build. Not only since it may be yout first build and you would be inexperienced, but my friend had a large sofa to use and was able to box the sub in with the wooden framing of the sofa.
You see, I'm all about concealment. You can't see any electronic device of my system. For example, I have my plasma tv embedded in the wall, including the cables. To further conceal my tv when not in use, I have a large Bob Ross painting to cover the tv. This also keeps it clean. You can walk in my family room and you would not think I even had a tv, let alone a home theater.