Almost any sub that has controllable crossover points and volume will do.
You'll want to set the sub crossover frequency to about 65Hz so it will have no output above 80Hz. In this way it will not attract attention to itself and you can thus place it where you get bass at the listening chair (do not assume that it simply goes between the speakers). This may take a bit of work, moving the sub.
A more ideal method is to use multiple subs, so as to break up standing waves in the room that are caused by the low frequencies bouncing around and cancelling themselves or reinforcing themselves. To do this you need four subs and they are placed asymmetrically in the room. Again, none of them will have response above 80Hz so they won't attract attention. The best subs for this purpose are made by Audiokinesis in Texas and are known as the Swarm.
You'll want to set the sub crossover frequency to about 65Hz so it will have no output above 80Hz. In this way it will not attract attention to itself and you can thus place it where you get bass at the listening chair (do not assume that it simply goes between the speakers). This may take a bit of work, moving the sub.
A more ideal method is to use multiple subs, so as to break up standing waves in the room that are caused by the low frequencies bouncing around and cancelling themselves or reinforcing themselves. To do this you need four subs and they are placed asymmetrically in the room. Again, none of them will have response above 80Hz so they won't attract attention. The best subs for this purpose are made by Audiokinesis in Texas and are known as the Swarm.