It all depends on what you are looking for. For modern speakers being driven by appropriate electronics playing anything other than electronica, EDM, heavy bass rap, or serious pipe organ music, REL makes some great subs that typically integrate well in most residential rooms. Two is usually a better choice to maximize the effects of room resonance, but one can be effective if you listen privately from the same position.
If your taste runs to the bass-heavy categories I described, and/or you want to feel the thump in your chest and have your pant cuffs flutter in the breeze, other options are available.
If your taste runs to the bass-heavy categories I described, and/or you want to feel the thump in your chest and have your pant cuffs flutter in the breeze, other options are available.