Here's the simple answers/problems/issues to deal with:
Easier to get good sound set up along the "long wall" for the inexperienced, and with less acoustical treatments. You want to start with seats, and try 1/3 or 1/5 possitions. IF you sit closer to your speakers with a "short wall"(sitting down the long wall facing the short wall) setup, you'll also have acoustical advantages in hearing more direct sound vs. reflected. Othewise, ACOUSTICAL TREATMENTS ARE MORE OF A MUST!(Compared to a long wall set up).
You want to get all your speakers set up in relation to your seats where you get FLAT FREQUENCY RESPONSE in the bass!(20hz-300hz reigion)...or as close as possible, making sure of response in critical crossover points. Also, exact phase between speakers and sub(s) is a must for coherence and balance. You can either move around the room and listen for best spots(then measure) while you play music through one speaker(summed stereo) that's placed where your head will be(or vise versa) for best bass response!(easy), or simply use test tones/discs and sound meter and measure for best response for EACH SPEAKER and sub!
In short, balance is Key! Once you get good seating possitions(I recomment maybe 2 to 4 at most in that narrow room(see old issues of SGTHT mag's "Home theater architect" for "set up/seating instructions"), you want to make sure the balance of soundstage and imaging is good...proper width/perspective in relation to screen for proximity, proper "toe-in" for tonality, "aiming" for soundstage perspective, etc. You will strongly be advised to do acoustical absorbtion or diffusion at the first reflection points on the walls/ceiling between your ears and the speakers if you do a "short wall set up", and better also for "long wall setup"...also, absorbtion behind your mains speakers(dirrectly behind mostly/only).
Your overall room balance is more absorbtion up front vs. more diffusion/reflection in the back half of the room. Things can get more involved with "reverb" and others, but consider more on the reflecton side to gard agains "over damping" and a dead sounding room. Still, with sread out absorbers around the front half especially, you should be ok, otherwise diffusors on the sides with some abosorbtion. If speakers are out more in the room, not so much concern up front with absorb/diffuse, so maybe a little behind speakers with either. (see HT mag's March issue(?) for their "echo chamber" article for custom HT)
Don't sit near walls, and more out in the room (again, consider odd multiple seating ratio's like 1/3's, 1/5's, 1/7's, etc. In relation, you want to look for even number multiple placements as starting points for speakers(1/6's,1/10's,etc).
This is all pretty accurate for a "closed in" room. OTher considerations for rooms with odd shapes/dimmensions and openings to other rooms/spaces. Still, you want flat/even response for all speakers and sub(s) from the listenging possition, (to hear more dirrect sound) to treat reflections which color and couple with the dirrect sound, proper soundstage width, tonality from toe-in, as well as "aim of speakers" for proper soundstage/height/perspective/etc.
Can you change dimmensions by moving walls? How about ading floating floor? If everyting is set there, a low pile carpet is good for you likely.
Basically, that's it! Otherwise, consult. for maxium results. But, doign what I said correctly will get you an easy 200% better results than most end up with ever!...and light years towards a top notch experience.
Most people don't realize how much all these variable add up, or WORK AGAINST YOU if you don't adress them! Small rooms like these are work!..but well worth it in your goal to make the acoustical enviroment and visual experience duplicate the experience as it was intended...special!...massive!..dynamic!...balanced!...uncolored...detailed...and accurate! Good luck...and don't forget proper bass mangagment...AMEN.
