Hi Mark,
I have a room of similar proportions but slightly smaller than yours and found that the QS8s sound best to me near the back corners with the tweeters parallel to the back and side walls. This puts them about 135 degrees back from the center of the screen which is a little farther than usually recommended. They not only sound better here from the sweet spot but also it gets them further away from the ears of those sitting on the ends of the futon.
I also noticed that in my room they sounded the same on just the stands or elevated about two more feet when I was using a flat bed for seating. However, when I got a futon and put the back up to use it as a couch they sounded better when up higher at about 6 feet.
If there is any way for you to experiment before finishing the wiring or leaving some extra wire to experiment afterward I recommend it.
The reality is you will find it difficult to put them anywhere that they wont sound great.
Dean
