So the peak of the ceiling runs from from front to back? And from the center peak each half of the ceiling slopes down to meet the walls which are 4' high? The 17' dimension is in line with the ceiling-peak line? And then the room is 13' wide?
Well if you bring your speakers into the room about 5 feet from the wall behind them and sit 8 feet back from the front plane of the speaker that leaves you about 3-4 feet behind your listening position. Separate your speakers about 8 feet leaving about 2.5 feet between the side walls and the speaker.
Because of your sloping ceiling you will need to deal with reflections from the ceiling near your listening position. I would think diffusion, rather then absorption would work best here. You will have a rather tight 8 foot listening triangle and the ceiling height will be rather low where you are sitting. This could be a real issue for you.
Perhaps I have your room dimensions wrong in the above situation? It would be far better, IMO, to have your listening position under the 8 foot peak of your ceiling.
In this situation your room is now 13 feet long in terms of distance from the back wall to wall behind your listening position. You could place the speakers 4 feet away from the 17 foot wall behind the speakers and sit just 1 foot away from the wall behind you. You will now be 8 feet from the front plane of the speakers. Place diffusers on the wall behind your listening position.
Separate the speakers 8 feet apart and they will now be about 4.5 feet away from the 4 ft. tall side walls. Perhaps separate the speakers by 7 feet (center to center) leaving 5 feet of distance between the side walls and the speaker. You will also need to toe in the speakers to avoid side ceiling reflections
If I have your room in my mind correctly, this would be a better set-up IMO. The ceiling slope area near the speakers would be another place to diffuse or absorb.
I don't think the room is ideal, but a challenge to be frank. The ceiling slope is the issue IMO. The second set-up may work with effort.
Well, I went back a re-measured:length is 17' 11", width is 13'7", side knee walls are 44" high then slope to a 4 foot wide flat mid line ceiling that is 8' high and runs parallel to the 17' 11'' walls. Room is carpeted over trusses. Walls are 2X6 and listed as insulated and sound proofed and floor is trusses listed as sound proofed.
My tweeter height is 43 inches and the speakers would likely be at least two feet from side walls (where ceiling height is about 5 to 5 1/2 feet) and speakers would be about 5 feet out from front wall