Add a subwoofer?

To improve the sound quality you would need to add two 15” subs and a two way crossover.

I disagree with this completely.  You don’t need 15” subs as even 12” subs will be more than adequate in your situation, and using the adjustments on a good sub and proper setup will be more than fine without an external 2-way crossover.  Could it possibly be better with bigger subs and an external crossover?  Yeah, but at much higher cost but you’ll still get very significant performance improvements without those things and to say otherwise is really just silly talk IMHO.  Try the SVS subs risk free as I suggested above and you’ll see just how much benefit you can get for relatively not much $$$.

I have owned Cornwall IVs and LaScala IIs. It is true that you need a big(er) sub to keep up with the 15” drivers. Although I ended up going with HSU,  the owner of Rhythmik steered me away from a pair of 12” subs. I bought a pair of ULS 15 II subs that paired well with both horn speakers.

I second the HSU's. Good value for dollars spent. Take a visit to their website.

The also answer their phone and talk to you.

Great info.  Thanks to everyone.  This will be a journey.  And moving furniture a bit I’m sure. Very cool stuff 

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.