2 to 3x's the width of the set back should be fine for most anything on that set. It will of course be better on 480p dvd and better material. I don't think I'd sweat anthing beyond this range for your size set. I mean, we're talking 30"!...not 60" plus.
However, the audio, that's another matter. Simply placing your couch on the back wall WILL YIELD you bad sound...that's garanteed. You have boomy unatural sound for certain. Really, you should start with your seating spot(s) at the 2/3's room distance, and the speakers at the 1/6 width/deepth possitions for 2 seats. If you do a stereo single seating position option instead, things change. Basically, it's whatever yields "flattest" frequency response, and maintains a good stage width/perspective.
I just have reservations about your sound quality, that's all. Your video, I don't worry about, because it's easy to figure for that...2 to 3x's the width is fine, or 1.5 to 2 x's the diagonal, and it's fine for most.
P.s., an easy trick most should use to figure best speaker locations, is to first get the seats away from the walls, more out in the 1/3's or 1/5 positions from width/deepth. Then, put speakers "bass woofer/port" where your ears will be. Then play some bass steady music, while you crawl around where you in the general area your speakers should go, listening for most solid, smoothest, even response. It will also measure right, and can/should be checked with a sound level meter and test tones.
The bass will sound fast, powerful, and accurate, not like a boom box. This should be done for all speakrs.
Good luck
