Hi Timk
I have exactly the same room dimensions and openings as you do! Square rooms are indeed very difficult for good sound.
I have the Harbeth SHL-5 and use as listening area practically half the room. Distance between the speakers is about 6,5 ft (about 2 meters)and from listening point 9 ft (2,7 meters) The speakers are 3,5 ft (about 1,1 meter) from the back wall. The result is very very good. I don't have any issues with the bass or the high freq, everything is OK! I have never had any better sound in my home!
Go for the SHL-5, it is a fantastic speaker and very easy to position.
You will not need the sub, that is for sure!