If the transition from a compression driver to a rectangular horn is less than optimal, where in the frequency band will this be most audible - and how?

I ask because I use a pair of large rectangular 200 Hz mid/high horns for a pair of BMS 4592 ND drivers (I use this combo from 266 Hz and up). The throat transition is not a disaster, but it is not super smooth either, and I have some harshness in the treble (6-9kHz, I think) and wonder to what extent this harchness may be due to the non-optimal transition. Thanks!

All the best
Peter