I would also first remove the bose surround back, not just because of timbre matching but also the single surround back is pointless, it is directly behind your head and with the way human ears are positioned (foward) its kind of pointless to have it right behind your head, i would instead move the sides to the the rear then you will hear it and it will give a more balanced sound (traditional 5.1), intead of loading all of the "big guns" in the front and sides you'd be spreading them out, think of it like this, your existing sound system would be like someone shouting on the sides and whispering into the back of your head, this way it would be like talking that would go to the sides and back. Then as far as the sub goes I would recommend trying to find a Velodyne CHT-15 due to low distortion, and extremely high output.