So actually this solution is technically superior to the D1500 filter switch?
Yes.
If there is a specific <80Hz sound from one channel you will only get that sound from one subwoofer and from the midrange/top speaker that it is Xover to. This gives the possibility of missing a sound because of one null point (placement) with only a single subwoofer reproducing the sound.
Agreed, but surely this is no different to a single pair of 8ks in a pure stereo environment?
Edit: Now here's a thought. I wonder if hitting a bass null from one channel in a stereo pair is actually important psycho-acoustically? i.e. The brain uses the missing information in its mapping out of the room?