Perhaps if you search something in the throat jungle you might fish some interesting results. You see, optimizing the driver to throat transition you might get some HF out of upper bass driver. But besides getting more HF there is something else in there. Let me to explain.
Pretend that you have a perfect upperbass driver with very light cone, no excursion and with an ability to do OK higher frequencies. Then still to get HF out of that upper bass horn might be complicated. The upperbass horn has LF equalization that from my perspective screw up transient response at HF no mater what we do. I am not talking about the fool what make bent horns. Even in context of a straight spherical horn that for instance does 100Hz and the driver could be pushed up to 4000Hz the sound at 4000Hz is not necessary good. The horn is just too deep for highest frequencies: the radiation pattern suffers and the transients of the HF suffer… not to mention that the 100Hz horn would be with dumped diaphragm that is optimized for lower knee. I feel that for better result at higher knee other horns would be more suitable. So, it is not about getting out of the upperbass horn more HF but rather to see HOW GOOD those HF would be. Most of the upper bass horns do not do HF well.
Perhaps a solution might be found in the optimization of a driver to throat transition. I personally do not think so as the premises that “the upperbass horn is too deep for given HF”, the basis horn geometry will not be changed even if you make the very good throat transition. Perhaps a solution might be found in use of compression drivers for upper bass horns. There is only one know to me driver that would be suitable for the task: ALE P1260, the one that Johan Dreyer used in his “Fundamentals Channel”. Another solution might be truing to inject into upperbass channel another overlapping channel with correct (or excessive) transition characteristics at higher frequencies. It is what I will be trying to do and I do not know yet if it will be working…
Anyhow, prior to any experiment with driver to throat transition we need to clearly identify what we are looking for. It is not about out intellectual satisfaction that one solution would be more “kosher” then other. We need to hear a specific horn. Then to identify what it dissatisfy us if any and then, ONLY THEN try to address this specific problem of sound. If it will be addressable by the driver to throat transition then it would be it….
Romy the Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche