I found a few 10" drivers that had response like that too. One that jumps to mind is the Eminence 2510. Response looked almost just like what you found with the B&C 8PE21. I decided it would probably sound good to 1kHz, but not past that.

Out of curiosity, did you measure in free air or was it done on a ground plane or against a back wall? The horn was designed to be used with boundary reinforcement, so if you measured in free air or with just a single boundary i.e. ground plane, you'll probably find response smoothes out a bit when the radiating angle is more constrained.