Cardas placement has to do with speakers that act as omnisirectional sources, as pretty much all dynamic driver speakers do to some extent up to about 9KHz. Horns don't really act like that. Horns aren't really wave guides. They're compression transformers. They lose their effectiveness once the size of the wave exceeds the dimensions of the horn mouth. That tends to make them vastly more directional than a dynamic speaker and allows for much more liberal placement. Cardas is intended to avoid boundary reinforcement and effects. You can't really reinforce a horn with a room unless you're using part of the room as the horn, like the Klipschorn.
At least that's my understanding of the things...
At least that's my understanding of the things...