first thought is that you need to place them where they SOUND THE BEST!(i.e, they are responding "flat" and "full sounding, balanced with the mains, etc). Then, the immage they create should dissapear as rears< create a large enveloping soundstage, have good width, etc.
For surrounds, I don't place as big an emphysis on "pinpoint accurate soundstage" as with the mains/fronts! Why?...because the front's where the specific "on screen action" is! It's where your "main focus should be"! The rears should not draw attention to themselves(which is what iwll happen if you sit right next to one that's pointed at your head/ear, too low, etc), nor distract you from the movie, and be banging away at your ear! Also, they should create an "unspecific image" largely" IMO. I know many will differ however. Some like solid detail behind them. To me, I find this distracting, "non-diffuse", to point specific and predictable, and simply "fatiguing" and tiresome over time. I just want the back info to be more "fill" and "ambiance".
I think at the end of the day this is far more enjoyable. In the front, I want it "tight as a rock" however!
Anyway, your other challenge is your problem with your couch agains the back wall. Sonically, you're sitting in a "boom box" of bass anomilies, and basic "unatural" sound basewise. In short, you are listening to a never ending "small room boomy sound" with your head at the wall! You might reconsider moving out at least a foot or so (AT THE VERY LEAST, IF NOT MORE!...OR RE-LOCATE and/or "EQ" the system...otherwise you'll never have superb sound.
To more specifically answer your question, that all said, I'd be looking to either put Di-pole's/Bi-poles on the ceiling, or "Dipoles" maybe way up high on your back wall. That way, either chioce will sound less localizable. And I'd first probably(as always) recommend "dipoles", as they dissapear better...which goes with my philosophy.
Although, they should be placed to sound as much like the mains as possible. Which sometimes means placing the bass drivers at the same plane as the main's, or the dirrect opposite(ie., if the main's bass woofer's are at 1.5ft off the floor, placing the rears bass drivers 1.5ft off the ceiling often works. But in your case, you have a vauled ceiling. Also, if di--poles, which are out of phase, you need other placement location ideally. So place for best sound(I like to play stereo through the di-pole and listen for best bass integration and "flatness of response", as well as integration with sub, etc.
Basically, you gotta experiment for best sound...put some work into it! There's no "pat-answer".
If it were my room, I'd be finding a way to get my head away from the wall, maybe enlist an EQ, be placing the speakers where they sounded the best, top to bottom, first and formost. Then, I'd make sure the rear surrounds weren't banging in my ear, distracting me, and dissapearing the best, with the best envelopment and effectiveness for their job.
I know many will like dirrect radiators "alway", but that's not the best for your situation I think. If so, You need them on the ceiling pointing down towards you more I think.
For you, I'd go ceiling, or Dipoles up high on the back wall, spead out, or otherwise right in the middle pointed oout! You might experiment. If dirrect radiators on the back wall, I'd fire them out probably out towards the ceiling or sidewalls to creat a more diffuse sound. If you had sidewalls that were closer and symetrical, and I had to go "dirrect radiators", I'd chose sidewall placment shoooting out either "criss-cross" across the room, or tinker with high placment on the sides firing in towards the middle. Again, tinker for best sound, and what you like. Everyone differs. good luck
