There's one way to get 90o directivity from a single LF source, and that's to put it into a corner. Other than that, a free-standing basshorn can't because it isn't large enough. Room corners can provide 90o directivity because the walls are large enough to direct the wavefront.

About distortion, I can always hear the difference between a driver with an effective shorting ring and I think you're right about the reason why. Even-order harmonics are suppressed well up into the midrange. Flux control rings aren't usually made big enough to do a particularly good job at very low bass frequencies, but they work great from midbass through the midrange. The power range I'm talking about is from about 1% to maybe 25% or so, maybe as high as 50%. For a 600 watt driver, between 10 watts and 100 watts, the difference between one with an effective shorting ring and another similar unit without a ring is clearly audible.

Merry Christmas!

Wayne