This doesn't always work, but its worth trying. Instead of placing your speakers along one of the walls, try putting them at 45 degree angles between 2 walls. To visualize what I'm talking about, your listening chair should be directly lined up with and facing one of the corners. From that point, just set your speakers up like you normally would in relation to your chair. You'll still have to fine tune speaker and listening chair placement. Just to be clear, your center image should be in front of the corner you are facing.
Speakers vs. headphones
OK, desperation is setting in (I'll admit it). I have a 12-by-13 room with 7-foot ceilings (carpeted room and acoustic tiles up top) and I've tried every speaker and sub placement imaginable for my very modest system (NAD 326 amp, Totem Mites, Velodyne SL-800 sub). Even without the sub, I can't get the sound to anywhere near what I'd like it to be. I had an even more basic system (NHT Super Zeros, Onkyo amp, M&K sub) in my old home and it sounded INCREDIBLE compared to what I have now. Like I said, I've tried diagonal speaker placements and everything else, including room treatments (panels on first reflection points – no bass traps yet, but it seems the problems go well beyond bass). The question(s): Is it time for headphones? Is it even possible to get decent sound in a room with these dimensions?