No need to slope the ceiling. What Mike has (and what I have) is a large soffit that goes around the perimeter of the ceiling. It makes a huge bass trap that is out of the way and adds architectural interest to the room. I suggest making a "shoebox" shape. This is very predicatble. If you start slanting walls and doing things of this nature then it will be dificult to predict what will happen once the room is built.
As someone pointed out, a room of this size will require a lot of speaker to drive it. I would suggest a high efficiency speaker. in a room that size I would not considre anything less than 92 dB 1Watt-1 meter. (Not the 2.83V thing) Of course horns would work well. But also Goebel if you are looking for a "box" speaker. The Stenhiem Ultime 2 is also efficient. You are likely going to need a very good pair of large subwoofers to get the low bass and really drive the room.
