Congrats on the new speaker Jeff. I like the slanted baffle. If it sounds good enough to deserve paint then my wife will think it looks better than any I have built previously.
Can you explain why there is ~1" space between tweeter and woofer? I thought that closer was better.
Is your active 50Hz highpass a 2nd order? With subs can Kairos play loud symphonies and rock, or should highpass be even higher for loud music? Any possibility of a 3 way in the future?
The 1" space is a compromise. I needed to achieve the proper offset, but didn't want to have a cabinet angle exceeding 15 degrees. If I spaced the centers fo the two drivers 6" apart (152mm) then I could achieve the proper offset with a 14 degree slant in the baffle.
The rule of thumb about keeping two drivers as close a possible is intended to reduce lobing issues. However, spacing also depends on the crossover frequency, and with the crossover point of about 1.9kHz for this speaker the extra space does not cause any real issues. The drivers are still within one wavelength of the crossover frequency and lobing is well-behaved.
In my active system the highpass is 4th order, but I also drive the speaker full-range too without any issue. With the low tuning frequency I would not push the vented version real hard driving it full-range. You can observe quite a bit of cone motion when it is vented. When I plug the port, which is my preferred way to listen to the speaker I can push it pretty hard without seeing / hearing any problems arise. Of course, this is still only a 6.5" driver and it will only move so much air, so it is limited where high SPL's are concerned. The speaker can still play pretty loud in my family room, but I don't know if my "loud" and your loud are the same thing. If you have a subwoofer and use a highpass filters, and plug the port, then you could easily drive it over 100 dB in room, which for most people is pretty loud.
I have not worked on a three-way version, and I don't have any plans at this time, but I won't say it isn't possilbe.