Suggest reading Floyd Toole's "Sound Reproduction" 3rd edition, it is the consummate audiophile primer for understanding how speakers/subs interact in-room. Toole worked for the National (Canadian) Research Council for decades before retiring from Harmon International. He invented Spinorama, an anechoic testing method that predicts in-room speaker performance. He and other acousticians like Earl Geddes recommend multiple subs (3 or 4) to help control bass peaks/dips that are inherent to all residentially sized rooms. The Rythmik L12 is a very good choice for use in a multiple sub setup.
He would avoid putting subs side by side and especially all lined up along the front wall with the mains. Sound behaves as waves up to the transition (Schroeder frequency - roughly 300 Hz). If you think of a bathtub with 4 inches of water and moving your hand in the water back and forth from end to end, waves are created. As the waves hit the ends they bounce back where they meet the next wave and either double, cancel, or interfere with each other. Now imagine a second hand doing the same from from the side of the tub. The waves would still energize the water but at a more uniform way. So using side by side subs would just double the bass peaks/dips.
I have 3 subs in my 8ft x 13ft x 21ft room (could do 4 but 3 is enough). They are set up along the walls (where subs work best). One is fairly tight to the front right corner, one is about 2.5ft from the front left corner, and the last one is roughly midspan along the left side wall. My mains are 5.5 ft from the front wall and 1.5 ft from side walls, severely toed in, with the chair in the middle of the room (mid-field). Also have six 2ft x 4ft GIK 244 full range panels, four 2ft x 4ft GIK 244 bass traps, three tall open randomly filled bookcases on side walls to act as casual diffusers, and use <500 Hz Dirac room measurement/correction software. Found that 60 Hz crossover sounded better than 80 Hz.
Toole recommends getting the right speakers first, then adding room treatments for bass second, although in his own home he violated that to maintain better views from expansive windows which he accepted as a challenge. Third step would be bringing in the subs. Finally apply DSP, but always best to solve physical problems via physical means (quoted from Earl Geddes). BTW my room, done independently, is almost identical to Earl's and follows Fibonacci ratios, ala Cardas. Toole gives several examples of 4 sub setups in his book. One of the best is centering them on all four walls. Another good one was in each corner.
