If you can add more Fountek ribbons later, I would vote for that approach vs. a full line of the PT2's. If you can not add additional tweeters, then I would suggest the PT2's.

Regarding comb line effects: I give some guidance (spacing, choice of driver type, the freqeuncy range coverage,etc.) in the white paper but the best answer is to build and measure. The white paper stuff is intented to get you in the ball park but ultimately the correct answer is to measure. From a design viewpoint you would be safest if you use a conservative critieria on spacing--like c-t-c spacing of less than a half wavelength vs. one wavelength.

Frankly, the Axon array isn't a high quality design based on the drivers used and the limited length and spacing of the tweeter line. Using either planar or ribbon tweeters would be much preferred to the large tweeters used in the Axon design. The 98 dB SPL value is clearly bogus or based on very optimistic assumptions. Maybe they have reinvented acoustical physics.

Jim