Interesting, I plan almost exactly the same ( finished dimensions of the room will be a little larger but not by much) same JBL's too but I have 2 JBL's 4645c subs which are huge and I hope to accomodate them somehow behind the baffle wall. I currently have the 3677's in partial baffle wall and smaller subs in my current room but they work really well. It'll be interesting to see how this works out for you.The LCR are JBL 3677's with L & R slightly toed inwards. The baffle wall will prob. be constructed :-
2:35 AT Screen 3.2m x 1.4m
Room 3.6m x 4.9m ( will be treated ).
Baffle Wall Construction :-
Ply ( 9mm min )
12.5mm Plasterboard ( Blue stuff )
20-30mm diffusion / absorption to resolve issues with sound bouncing back of AT screen.