You are probably better treating the floors (and possibly the walls) rather than the ceiling. I have a similar room. Is the ceiling higher above one speaker than the other? Can you place the system on the wall under the low end of the ceiling so the speakers are firing down the long end of the room? This will provide better balance and maybe less trouble with reflections.
At one time I had my system set up similar to what I described, and it sounded much better after I moved it 90 degrees to fire down the length of the room.
JD
At one time I had my system set up similar to what I described, and it sounded much better after I moved it 90 degrees to fire down the length of the room.
JD