I usually prefer long wall placement when it's a real option.
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Long wall is best for many of the reasons above, especially the sidewall first reflection issue Tom mentions.
HOWEVER, the short dimension of the room must be 15 feet, bare minimum: 2.5 ft minimum behind the speakers which are 7 ft apart (midranges), + 1.5 ft speaker cabinet depth (average) + 7 ft from the front of the drivers to your ear (when seated) + 1 ft chair or sofa back, + 3 ft for bookshelf (abfussor) and walkway behind sofa. Total = 15 ft, and the long dimension must be 21 feet or more.
You can try and eliminate the 3 ft bookshelf/walkway, but even with absorption on the rear wall (behind the sofa) if you shove the sofa against the rear wall, the bass will be pretty boomy and you won't hear a good soundstage. So I'd recommend you go for the short wall. 13 ft is a decent width (12 ft or less is a bitch!) and you will need sidewall treatment to kill the first reflections. You can pull the speakers out 5 to 7 ft from the rear wall (a very good thing) and put your ears about 7 ft back from the drivers, leaving you 6 to 7 ft behind your (ears) listening position.
The reason for 21 ft being the minimum long dimension, is that with speakers centered on the long wall about 7 ft apart, the path length for the first reflection from either speaker to the sidewall and back to your ear is approximately 17 feet. Since sound travels approx. 1100 ft/sec, it takes the reflected wave about 15 milliseconds to reach your ear while the direct sound, travelling 7 ft, takes only 6 milliseconds, a difference of 9 milliseconds. As long as the difference is at least 7 milliseconds (if my memory still works ;--) your brain will hear the reflected wave as an echo (which is OK) but if the difference is less than 7 milliseconds, your brain will combine the direct and reflected sound as if it was ALL DIRECT SOUND and that's what SCREWS UP THE IMAGE AND SOUNDSTAGE.
I am currently set up pretty much like Bob suggested, even more into the room (7ft), 28" from side and I listen about 4ft away from the back wall so the depth is great and I pretty much removed the room from the equation (short of the low ceiling). Both first reflection points are treated with Acco fiberglass panels. I agree that my coicnern is that 13ft for short wall is not enough to try long wall placement, especially with thes espeaker which project deep behind.
Nsgarch has a valid point for keeping the speaker well away from side walls.
As long as the difference is at least 7 milliseconds (if my memory still works
Bob Katz (recording engineer for Chesky) recommends about 5 millisecs....so in practice between 3 and 4 feet of distance between speaker and side wall is a good rule of thumb.....if this means you need a long dimension then it may be the way to go.
My room is just about exactly this dimension, and after a lot of playing around I have ended up on the short wall with the speakers pulled out about 3 feet from the back and side walls, and about 7 feet apart. I sit about 7 feet away, so basically near field listening. Slight toe-in. Imaging is great.
I do like the long wall idea but I agree with the poster above that the room would need to be bigger.