Somehow, I don't find Ian's observation contradictory. Sound emanating from a linear source (BC) drops slower compared to that from a point source (conventional). So, to observe the same kind of drop you need to move further hence the expanded sweet spot. Basically, due to slower drop of sound strength with distance, all the effective physical dimensions (separation between speakers, where you sense the sweet spot, where it ends, etc..) get expanded too. ...just the way I see it.

Shiral