Sorry for missing this.
I certainly understand the desire to preserve the view but that's absolutely the best way to set things up too. If the flat screen were on a low stand, you'd still have a lot of the window available.
If it has to stay as is, I would do something much thicker on the wall behind you. Behind the speakers, one can either use absorption or diffusion. I prefer diffusion for the rear wave personally.
The left side of the room in the current orientation is going to act like a huge horn. Some bass control on the side wall where the stairs are not would be a big benefit. Since it's not terribly usable anyway, something like Soffits in the corner and across the floor would be good.
Bryan