Good question. I've seen folks advocate having the speakers on the wall with the high ceiling, and that may be so, but I would be concerned that your seat under the sloping walls would put you in a position to pick up early reflections off the sloping part of the ceiling. I suppose much will depend on where you put your chair. If the ceiling just in front of it is 8' it probably wouldn't make much difference.
In my last home, which had a sloping ceiling as you describe, in a slightly smaller room, and with the speakers closer to the wall under the sloping ceiling, I could never get a great sound. I remedied this by putting the speakers on the long wall firing accross the sloping ceiling and it worked fine.
Best to keep an open mind on the possibilities and try all four walls.
In my last home, which had a sloping ceiling as you describe, in a slightly smaller room, and with the speakers closer to the wall under the sloping ceiling, I could never get a great sound. I remedied this by putting the speakers on the long wall firing accross the sloping ceiling and it worked fine.
Best to keep an open mind on the possibilities and try all four walls.