Greg, I understand the Tractrix front-loaded horn arrangement just fine, and there are some things like that already out there using Lowthers, like the Oris 150. However, most designs like that have a box loading of the rear of the Lowther, and I was most interested that your design did not load the cone in the normal manner.
I expected the rear radiations to play into this equation at some point, and was interested in your comments about that. I think the idea of an open baffle Lowther is interesting. It sounds to me like you ended up with some sort of "aperiodic" loading design.
To Paul: The horn contour is designed to have a "lower cutoff frequency" which is defined by the length and mouth size of the horn. At this "lower cutoff frequency" the bass sounds are no longer amplified by the horn, and the efficiency drops off correspondingly, like an acoustical filter.