I would add that the brilliance control is a power variable resistor – in fact a resistive wire wound potentiometer.
This one is an 8 ohm unit and was taken from old A1’s – we can see that the operating zone of the wire had became very hot from the color change of the wire. In my speakers I get flat balance with 2 ohm – using six 12ohm Mundorf 10W resistors in parallel.