This is the main problem with dipoles. My suggestion is do everything possible to cancel or diffuse that rear wave. Bring the speakers out into the room as much as possible and then put something behind the speakers to deal with the rear wave. The rear wave is the problem and you cant possibly bring the entire bandwidth of reflected sound into proper phase.
You dont want your speaker doing anything other than reproducing what it is fed as accurately as possible. So the shape of an instrument should nothing to do with the shape of the transducer reproducing it. This is why I always get irritated when speaker manufacturers talk about making their cabinets out of the same material as instruments.
Dont worry about subwoofers and damage. Just be sure to cross your subwoofers over at the lowest frequency possible. Probably around 40 to 55 hz.