Subs always help unless they aren't set up right. It sounds like you want to do it right, which means filling in low end information that the speakers can't reproduce.
While the Cornwalls are very efficient, it's not going to be hard to "keep up" with them as some others have said. I have Klipsch Chorus and Quartets. I have a Rythmik F15 with the Chorus and the volume level is barely on.
You could easily get by with a moderately powered 12" sealed sub. My first recommendation would be Rythmik, but SVS has some very affordable models that would work. Two subs are better than one, and 12" sealed subs don't take up much space and would be much better than getting a single 15" sub.
If you want to pressurize the room and feel your bowels shake, you'll need to step things up, but if you just want to fill in the bottom end, you'll be fine with one or two small subs.