Due to the limited space, and since I already have all in-wall & ceiling surround speakers, I want the in-wall subwoover over the standard one. Also, I'm assuming they are passive subwoovers, so any recommendations for an amp to accompany it would help. I using an Onkyo TX-NR609 receiver. I have access behind the wall (this is also where all my equipment is - receivers, etc.) so it'll be pretty easy to connect.
That would be my suggestion as well but I'm not sure $500 is quite high enough to get one done and I'm not sure he has any interest in diy. Though maybe he could find some decent drivers with closeout pricing(I haven't looked lately). It would definitely outperform any commercial in-wall sub.
I've had good success with Polk in-wall and in-ceiling speakers. They even have an in-wall sub, I believe. Can't remember the model #s off hand, but I listened to 3 or 4 brands and thought the best sound for a modest cost was the Polks.