SCm:
Dipole sub does not mean "more" bass, and therefore it is not an overkill. However, a pair of dipole subs, which is what I have, may be an overkill for a small room. BTW, 14 by 18 is a not really small.
Personally, I prefer Stereo subs, if at all, even though my big Apogees generate enough bass without even a single sub in my 16 by 26 room.
Good luck.