I simulated the horn in Hornresp before making it and didn't see any problems, so I started there. And I intended to cut the horn down and re-measure, so it made sense to start with the longest horn first.

Above is a measurement of a slightly shorter(9" deep) version of the Pi horn which has been cut down, and I removed the back of the chamber. The response looks a lot better, though it may be overly smoothed due to gating.

I also have some measurements of an even shorter(7.5") horn I'll post later.

My impression is that the dips and peaks in the response are due to poor mouth termination?