Subwoofer Choice

Audition the RELs and put your hand on the enclosure. You will feel it shaking. That is distortion. Audition the Martin Logan and KEF balanced force subs. When you put your hand on the enclosure you will feel almost no shaking. These are significantly superior designs. The KEF even has a full two way crossover. Magico uses a similar design but they are very expensive. 

You have a fairly large room. My room is also 16 feet wide and I use 4 12" subs. The absolute minimum is two 10" units. I would not go any lower than two 12" subs or equivalent. You will also need a complete 2 way crossover. 1/2 the benefit of using subwoofers is removing the bass from the main speakers. This allows you to use a higher crossover point and significantly lower distortion in your main speakers along with adding up to 6 dB of headroom. Most commercial subs only have a low pass filter. You can high pass the main speakers by soldering a capacitor of the right size in series with the input. The capacitor value depends on the input impedance of the amp and the 3 dB down point (crossover point) you want to use. C = 1/RF Digikey will sell you any capacitor and your amp's manufacturer can tell you the input impedance (R) of your amp. 

@bound4h , The JLA crossover is a good analog crossover however the best sub management systems/crossovers are digital. Check Out MiniDSP which makes several reasonably priced units. They also have room control features, Dirac Live I think. There are now several digital preamps that have sub management, room control and EQ built in, MiniDSP, Anthem, DEQX and Trinnov to name a few. 

All subwoofers that have only one driver are compromised by Newton's third law. Sub drivers are heavy and play low frequencies at higher volumes will cause the entire enclose to vibrate/shake. This creates distortion and compromises dynamics. It does not matter how heavy the enclosure is. You just raise the frequency the enclosure shakes at. Some manufacturers have taken this into account in newer designs that use two opposing drivers that balance each other eliminating this shaking problem. KEF, Magico and Martin Logan all make "balanced force" subwoofers.