I have a room almost the same size as the OP's and have experienced very similar problems in set up. I too now have my speakers on the long wall. I started with them on the short wall and after almost wearing out the carpet from moving them around so much, I finally tried them on the long wall. Better this way but still not entirely satisfying. I have numerous room treatment pieces and have gained some improvements by using them.
Personnaly I think that the house may be improperly orientated with the earths magnetic fields thereby causing interference. Perhaps a visit from a Shaman to do a purification is needed.
Listening near field certainly provides the most clear, clean and detailed presentation but feels unnatural for me. I am more used to and prefer a more mid to far field position.
I contacted GIK for a room analysis and after providing them with drawings, photos and conversation they replied with responses such as "well you could do this, or that". Not really what I expected from them as I though they would provide a more specific "road map" of how to treat my room.
So I am still in the same boat as the OP and am interested in specifics from others that have had a similar problem and how it was solved.