You said you haven't tried bass traps. Bass traps transformed my room, 10x13x8. I finished it off with 24 1x2' SoundTrax Pro acoustic panels through out the room. Twelve on the back wall behind the listening chair and twelve on the side walls, six per side, extending forward from the speakers. The front wall has a window with a curtain and two of the bass traps centered vertically in the corners. A third is mounted in the corner of the the wall-ceiling juncture on the back wall. I made my own 2'x4'x4" bass traps out of 1x4 lumber and stacked 2 inch thick 2'x4' Rockwool boards. The bass panels were done last and had the greatest effect. The room was unlistenable until I did all this.
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?