One other thought on positioning subs after studying your room. If it was me I would first start with postioning 2 subs symetrically on either side of your centre channel. That is where you have the best chance of ensuring smoother FR and phasing with your mains given the large open space you have. With dual subs in a leaky space you have to concern yourself more with integation with your mains and locating close to those speakers on the same plane will help greatly.

John