hifi28 - your speaker positioning will be experimental with all things considered in your room.
An openable window can be a huge asset in that centered position. Sound waves through air accumulate as pressure zones in the corners and centers. Opening the window centered between the speakers might alleviate that pressure zone. Draperies between those cabinets might also be beneficial. The back end of the room benefits the most from absorption. Consider venting pressure from the corners, especially at the floor because the acoustic ceiling will reduce pressure up high. With care you can reduce standing waves to a small fraction of what they would be in an enclosed space. Well worth the effort.
For the record, most of the 'speaker problems' generally recited are not actually speaker problems, but rather room problems that may be excited more by some speakers than others. Thiel is capable of exciting problems due to their broad polar pattern and accurate response.
