[bpape]

If it was me, I'd just build another pair of what you already have and place 1 in each of the room corners. I'm not a big corner placement guy but when you can hit all 4, it's been proven to avoid some of the modal issues just as you're looking to do. If you consider each tower as a 'speaker', in theory, you should get an additional 6db from each tower so a total overall gain of 12db as you want.

Bryan