95wbch, is your room going to be a separate building? I ask because it influences ceiling shape.
A friend built a small recording studio on his property that was pentangular in floor plan and had a peaked roof. A regular pentagon was chosen because it somewhat simplified the building of it, in that the roof was simpler. The walls all leaned out about 15 or 20 degrees. It looked neat and sounded great. It was pleasantly reverberant, but had no objectionable eigentones. It was nice to be in.
Sloped-out walls may be impractical for most, though, and are not all that necessary. A shortened variation of the coffin shape could work too, with a cathedral ceiling.
In my case, I had to build within an existing space and it was appropriate that the overall shape be rectangular. My solution was to make the side walls (which I had built) curve inwards with a 14 foot radius. They were built, surprisingly perhaps, out of tempered hardboard bent around a frame of two by fours nailed to curved 3/4' plywood plates on the cement basement floor and similar on the floor joists above. In spite of being lightweight they were amazingly rigid and did not resonate at bass frequencies due to the curve. These curves also created a constantly varying room width, eliminating any transverse standing waves.
Behind the speakers I built a false wall which contained a giant widerange trap from the ceiling almost to the floor and extending outside the left and right speaker positions. The ceiling from the front wall (behind the speakers) extending out into the room about 7 feet was covered with acoustic foam to deaden the area and absorb the first reflection from the ceiling.
The results were gratifying, with very even bass throughout.
