I have a simular sized theater that I am working on in my house.
My Dimensions are 9'wide (for the first 5 ft. then 11 wide after) and 17' long w/9' ceilings. (edited)
So far so good, some of the things that can help ....I have M&K 150s (angled version for L&R) which are sealed and are limited to 80hz in freq. depth, both front side walls have (by the speakers) that GOM covered acoustic fiberglass (Acoustic Solutions brand) to absorb first reflections and then some, I am ordering bass traps for both rear corners of the room to cover from ceiling to floor.
I think for smaller rooms more absorbtion is needed than a larger room but I am no expert, I am close to posting pictures for more advice from the experts on this board. It doesent sound that bad, but I have not enlisted help yet to see what it can do....may be that it is terrible and I do not know it.