Hi Ian,
I will agree with You that is why on PT-H95HF and M2 horns such slot size is only 3/4-inch width somewhere in the throat part of the horn.
I think that using JBL D2430K, or BMS4592 would have a great problem to find proper horn that would be good for all applied frequencies
from the bottom to the top. May be 476Be would be "in" the same problem.
regards
ivica