If you can wait, The new Chane series 7, an all sealed design, should be an excellent option. ETA still to be determined. It will give you high efficiency and power handling, excellent dispersion characteristics, dynamic sound, exceptional vocal clarity and no listener fatigue.
In an 18 x 24 room (assuming it's sealed), you could get away with a single subwoofer. However, if you have multiple rows of seating or a large couch with many seating positions, it will be more difficult to get even frequency response with just one sub, especially if you have a predefined spot where it is going to go (the sub tells you where it goes, not the other way around).
So, if you are seeing your situation in this description, I would seriously recommend you find a way to include two subs in the room, not just one. Front and back, kitty corner, side to side (opposite each other, somewhat offset) is usually a successful approach.
Last edited by RayGuy; 08-19-2017 at 02:10 PM.