Another advantage of a rectangular horn is that you can go to a TMM (or would that be HWW?) format and get vertical as well as horizontal symmetry. I tried modelling symmetrical MTM (WHW?) cnofigurations but unless the crossover is quite low (or the horn too narrow in the vertical plane to maintain reasonable pattern control) the woofers end up too far apart and their vertical lobe is too narrow, in my opinion. Of course you'll still have the inevitable vertical pattern "pinch" at the crossover frequency.

Have you done any dual-woofer speakers? I don't recall seeing any, but then presumably not everything that happens in the lah-BORE-ah-TORE-ee enters your product line-up.

By the way, best of luck with your heat-pipe patent!

Duke