Years ago I had my system in a room of similar dimensions. The sound was always flat and uninvolving when I sat on the couch on the opposite wall. I did exactly as you are planning with a portable chair. made all the difference in the world.
After that experience I am a convert to nearfield (6 to 8 ft) listening. Let us know your impressions after the trial.
After that experience I am a convert to nearfield (6 to 8 ft) listening. Let us know your impressions after the trial.