Why can't ESL's be sited close to the rear wall? Is the problem to do with sound reflections? If so, surely you could place something super-absorbent as close behind the speakers as you like, as the sound won't be reflected back. Something like large picture frames with something thick and woolly attached. Then you can hide as much absorbent stuff behind those panels as you want - it's just a dead space anyway. Should be cheap and easy enough to do, then try an A/B comparison with the out-of-phase thing.