I have a "coffered ceiling" listening room (wherein the walls are parallel for a certain height & then slope upwards to the ceiling. It's in my attic area hence the shape of the walls. While it might be more diff. to calc. room modes, I think that it helps, overall, in reducing the room modes as there is less parallel walls area. Thus, I also feel that coffered ceiling rooms can be more easily treated for resonances than regular rooms. I have to admit that I have not tried my present system in a regular (non coffered ceiling) room.
It is the not the 1st time that I have seen the 2 books suggested by Ozfly to be recommended. I have been recommended the same by a very reliable source. I have been very lazy & have not read them! However, I do believe that they will provide you w/ some really good info that you can execute on with regular/easily available material at a DIY store.
Thus, it is my personal opinion from personal experience that your room will be just fine for listening. FWIW. IMHO. YMMV.
