Single vs. Dual Subs

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. 

Hi,  I have a similar question relating to 2 vs 1 subs.   I currently have a single REL sub sitting right next to my right speaker.   If I were to add a second sub.  Does it need to be placed symmetrically to the first sub?  i.e. right next to my left speaker.

The reason I ask is I simply don't have the room to accommodate a 2nd sub on the same side of the wall where my speakers are.   So is it ok to place the 2nd sub somewhere else, like on the opposite wall diagonally from the first sub.   Will that ruin my stereo imaging?

Thanks very much.

xcool - Our ears aren't good at locating low frequencies so subwoofers don't contribute to the stereo image. In fact you will probably find that the best locations are not near your speakers or symmetrical.
What you're looking for is for each sub to excite different room modes so that they balance one another out to some degree. If you're not familiar it might be worth looking up 'room modes' and 'standing waves'. There are also some neat tricks for finding good locations although the whole process is a lot easier with a measurement mic and some software. You need to also make sure that the speakers are 'time aligned' which you can do by measuring and with some simple maths.

It depends on the room, the mains and most of all, fc and slope.

If fc and slope reach into the lower mid-bass, one sub is localizable and must be placed between the mains.

IMO, 2 subs are mandatory for 2 channel. 3+ is a crowd.