I'm with Ivan on this one: try the long wall first. My room is close to the same size as yours and I use the long wall as well. My speakers are about 10.5' apart and 1.5' out from the front wall and I sit about 7' back with my head very close to the rear wall.
What Ivan says about the comb-filtering effect mirrors what I've read about it. It goes something like this: if the distance from the back wall to your head is less than the circumference of your head (typically 2') than your brain tends to ignore most reflections (if you're ever lost in a cave while spelunking, stand as close to a wall as possible making sound location easier).
I have great side to side soundstaging with great extension forward the speakers and just average depth behind the speakers (there's always a trade-off) depending on the recording.
If you don't like it, try it the other way before you nail everything down.
All the best,
Nonoise
What Ivan says about the comb-filtering effect mirrors what I've read about it. It goes something like this: if the distance from the back wall to your head is less than the circumference of your head (typically 2') than your brain tends to ignore most reflections (if you're ever lost in a cave while spelunking, stand as close to a wall as possible making sound location easier).
I have great side to side soundstaging with great extension forward the speakers and just average depth behind the speakers (there's always a trade-off) depending on the recording.
If you don't like it, try it the other way before you nail everything down.
All the best,
Nonoise