I have my JM Reynaud Offrandes setup in my room in a similar situation. The room is 16x21 overall but I have to setup with the speakers firing towards the short wall with only using one half the width of the room. So one speaker is about three feet from the wall w/window and the other has about 10 or so feet to the wall.
One room treatemnt that helped me get better imaging and soundstaging, basically just snapped everything in place better was the addition of Argent room lenses. I have one flanking each speaker and then one behind the speakers in the middle.
So basically I just diffracted the side wall reflections and then focused on treating the surfaces that I could. For me that meant a thick carpet on the floor, a 2'x6' ceiling absorber using Auralex foam, and an Echobuster Double Buster on the wall behind the listening position.
