I actually confused the notch filter and shelving equalizer which both are usually required for ESLs. The notch filter is used to dampen the large resonance peak of the membrane and a shelving equalizer is used to flatten out the high end. Due to dipole cancellation the ESLs frequency response is rising towards high frequencies. But because ESLs are highly directive and most sound arriving to the listening post is direct sound, it doesn't get mixed up to room reflections like a regular speaker. Therefore the sound will be overbright unless the natural response is shaped more like the power response (overall radiated sound in the room) of a conventional speaker. A shelving equalizer will do that - the tricky thing though is that this is usually only found in the high-end active amplified models. Meaning 4-5 zeros in the price tag.
If you've heard an ESL from starter level, it's been passively filtered. It may sound excellent, but it can't hold a candle to an actively filtered one.
Interesting, the EV's I had came with a frequency slope equalizer and it was 10k hz,IIRC,it had 3 setting -3db -6db and -9db,this was to help with as you say overly bright sound.
What ESL panels do you have? I've heard a couple of highend units before,as I said I know a guy that only sells "audiophille" equipment. I helped him with a couple of rooms,supplied the cabinets to "build in" a few different theater rooms.
Now this was going back about 20 years so many things have changed but I pop in once in awhile to try and keep up....
Jus need to leave the wallet at home when I do!