I'd be especially leery of an extremely small sub with an extremely large built in amp. As Patrick stated, Hoffman's Iron Law dictates that a sub can be small or sensitive and play low- but not both. You can get low bass & high volume out of a small box only by running in a LOT of power. This puts a good deal of thermal stress on an amp and a lot of mechanical stress on a driver. This can result in a tremendous amount of vibration being transmitted to the amp. To me, this is a recipe for premature failure, and before I went to passive subs with outboard amps, I left a trail of burnt, smoking cubes in my wake.
Siting the sub close to your listening location can sometimes help. Volume drops off at a rate inverse to distance (although there's still some room gain), so putting the near you can increase the perceived power. Sounds like your location is predetermined, though, so that may be out.
I agree that buying a sub with a built in parametric EQ (or adding one later) is almost mandatory. You can do without, but unless you get a serendipitously wonderful interface with the room by blind luck, you'll never know how good your bass can be til you EQ it. The Rocket sub has a built in EQ, as does the Infinity line of ROBAS subs. The Infinities have gotten good reviews, and if one will fit where it must go they'd be a good choice.
