There is a high frequency "hole" in line with the center plug. Its very HF but very noticeable with pink noise.
What if I replaced this horn with the H290C? I would need to figure out how to mount the H290C from the diaphragm side and trim off the mounting flange. Assuming I can pull off a nice solid mounting scheme, is this a bad idea for reasons I'm not thinking about?
It's really hard to say how it would sound. But I'll be honest, I'm afraid the flare will obscure too much of the midwoofer, creating an on-axis hole below the crossover point.
You can always get one and try it. Or maybe a better idea would be to make a 11" x 6" panel out of wood or cardboard, and place it in front of the driver where the waveguide would be. Then measure the midwoofer response with and without the panel. Pay attention to the region between 500Hz and 1kHz. That will tell you how the obstruction from the waveguide affects the midwoofer radiation pattern, and will let you know how the system will act in the frequency range just below the crossover point.