Wanted to give an update...
After lots of testing at high volumes, I've come to the conclusion that my current design doesn't meet my HT goals. There's way too much distortion generated by the horn when the Alpha 6s are driven to around 119db/meter, which is the volume I need to reach reference at my listening position. So far, I've tried everything I know to lower distortion, including the following:
1) putting a 1/8" spacer between the drivers and the horn body, thinking that the cones were rubbing against the wood at their max excursion- this might have helped, not sure yet. Xmax shouldn't be more than 1.5mm or so at 300hz at 119db. This lowered the upper f3 by about 75hz or so.
2) adding an entire sheath of wood around the horn, with green glue between the leaves, to deaden internal resonance. This has helped- the cabinet did ring quite a bit when I hit it before the modification, and now it makes more of a "thud."
3) added metal surrounds to the backs of the Alpha 6s to create stiffer sealing, thinking that the cardboard/tape combo was letting air out, which might have caused turbulent noise. This lowered my lower f3 by about 75 hz.
During my testing, I put my hand into the horn and closed off ports while playing at 119 db at 300hz to check the effect, and man, you would not believe how fast that air zooms in and out of those things! I modeled a 4th order bandpass box in WinISD to try to match those created by the Alpha 6s and the horn body, and low and behold, the port velocity at 300 hz, 119db is something like 100 m/s! Also, I think there's some buzzing/humming that's caused by the ports at certain frequencies- the port edges are so thin that they might be resonating.
So, I'm going to stretch the ports out along the radial axis of the horn to get the port air velocity down and also try to reshape them to make air flow more smooth. My fear in so doing is that I will create more diffraction generating interference for the higher frequencies handled by the compression driver. At this point, this project has become one of aerodynamics, and I don't have any formal training in this field. All I can do is guess at proper shapes, test them, and post results. Hopefully, I'll have something positive to report in the near future.