Doh!

You may have noticed that the mid range and the tweeter are too far apart. I completely ignored interference fringing around the crossover frequency. The units need to be separated by less than the wavelength of the crossover frequency - and they aren't. So as you move vertically (perpendicular to the axis), the waves from the two units interfere constructively and then destructively. Doh! This is something I really should have thought about. So I will be building a new baffle with the two units mounted much nearer together. Good job it's only screwed (sic) rather than glued. Oh, and I need to get a new amp or repair my old SUV6 which is cutting in and out on the left channel. A cheap valve amp would be nice but probably out of reach.