I see, this is very different question then you initially posted.
Ironically the subject was discussed years back what I was fighting with S2’s “fuzziness” at very top.
I still do not feel that there are any 'rule of thumb' in it, there are rather many “depends”. If your MF driver in your particular horn has problematic top end and you would like a bit roll it of then there are many options to consider BEFORE bringing a new amplification channel for HF. Here are a few options:
1. Use a deeper hornRgs, the Cat
2. Use a horn that has higher “constant directivity” characteristic
3. Introduce a very mild serial inductance to MF driver. (HF will need own amplification)
4. Mild application of damper on the surface of the MF defrag (like aquapluss)
5. Changing reflective characteristics of back chamber on MF driver
6. Lower the output stage that drives MF loading (the more effective and proper in my view)
7. Use HF channel in a way the it will mask out the upper MF driver
8. Reduce effectiveness of phase plug
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche