Sure Massimo,
The continuously adjustable brilliance control is basically an attenuator for high frequencies.
The frequency response of the panel (not considering any room effects etc) is designed to be linear or flat with bass and mid set at 0db and the corresponding 0db or flat response is having the brilliance set at 3pm.
This is not really indicated anywhere.
If you turn the brilliance fully clockwise to its maximum position, the contact in the control are actually bypassing the attenuation mechanism and you are hearing the high frequency the same as if there was no brilliance control (attenuation) at all. Note that if there is no attenuation, the impedance does fall to less than 2 ohms at the highest frequencies.