Single vs. Dual Subs

One Sub does not have the power to load most rooms down to 20 Hz at least up to 15 inches. An single 18" sub might get you there. Two 12's is the minimum. Two 15's even better.

Speaker frequency response is taken near field usually at one meter from the speaker. 4 meters back in a 16 X 30 foot room is a much different story. In order to get the right "feel" you need a rising response from 100 Hz down to 18 Hz. That takes big drivers and a lot of power. Two subs will also smooth out the nodal behavior in the room. Without proper low end you will never feel as if you are at a live performance.

carlsbad, I have been using ESLs for over 40 years and am very familiar with the CLS. When you get the second subwoofer get a two way crossover. Rolling the bass off in the CLS's will result in a very large improvement in distortion and headroom. You will easily get another 10 dB out of the system. Cross up between 100 and 120 Hz. I would use a Mini DSP to do this. That way you can use steep slopes and you can time align the subs to the CLS's which is very important. You will soon realize that the crossover is the single most important piece of equipment you have. Also, do not put the subs behind the CLS's put them outside the CLS's preferably in corners right up against the walls. They are an easy 6-9 dB more efficient in this location, the sub does not have to work as hard and this lowers distortion. 

Everyone needs to go to another live concert and ask this question (assuming the sound system is a good one) Why does this sound better to me. Why does this feel better to me. Jazz in a smaller venue is perfect. The idea is to recreate that sound and that feel in your room. Having a room longer than 30 feet is a big help but not mandatory. A lot of systems have the capability of doing this. What is usually missing is the bass. It requires a lot more than people realize, a lot more speaker and a lot more power. I just get there using 7000 watts and four 12" subwoofers. It requires a 2 way crossover with steep slopes and the ability to delay speakers to get them aligned correctly. Just adding bass is not the point, it is getting to realistic bass. No speaker that I have ever heard had realistic bass down to 20 Hz, not a Wilson, Magico, Magniplanar, or Apogee. It takes a specialized driver in a very heavy stiff enclosure and a lot of power. You could use one really big one but IMHO it is easier to live with multiple smaller ones than one huge one. In my case it is absolutely necessary. IMHO if you are going to add subwoofers wait until you can do it right. It can be very frustrating if you don't and waste a lot of your time.
@erik_squires , forget about simulations. Measure actual setups and see what they do in reality. A simulation can not possible cover all the variables involve like what frequency the walls resonate at. Are the subs well matched or are they 90 degrees out of phase with the main speakers. Why do some people think their subwoofers sound better 180 degrees out of phase? One sub may do fine in an 8 X 10 foot room or as fine as you can get in a room that small which is not very. Do not make up your mind on someone's idea of what a simulation should be like. 

@m-db, The much better solution is using digital bass management which includes room control and digital crossovers which are far better than analog crossovers. From less to more expensive you have MiniDSP, DEQX, Anthem and Trinnov. Check them out. If you are a person who is stuck on analog only just forget about it, my mistake. 

divertiti, It is impossible to perfectly tune a single sub so that it's frequency response is flat at any listening position in an enclosed room.
So, you can live with a single defective subwoofer or minimize the damage by getting two. Depending on your main speakers two may be enough.