Compare this with companies who make their +/- 0 dB setting a bypass circuit.
They do this because their circuitry is not as good (we are talking about much less expensive equalizers now).
Every time you move one of their filters, noise is introduced to the system in a cumulative effect.
Even when a filter is cut, the noise rushes in. This is probably the major price difference in analog equalizers.
Parametric EQ & Bowling
The ideal filter would be a digital parametric filter for control and low noise.
Parametric filters let you vary the Q any width you want from many octaves wide, to as narrow as 1/70th of an octave.
And, believe it, those feedback and ring modes are even narrower than that.
The idea of ridding feedback with a 1/3 octave EQ in a fixed installation system is like rolling a bowling ball to knock down only one of twenty adjacent toothpicks.
You get the one you want but you crush the rest. A 1/3 octave equalizer can diminish the feedback and destroy all the music and sound of the neighboring frequencies.
A parametric equalizer can selectively “notch” the offending frequency without harming the overall sound.
We now have much higher quality digital signal processors with built-in parametric equalizers and even stand-alone digital parametric equalizers for reasonable prices that operate very quietly.
Fixed installation narrow-banded EQ is available to anyone on almost any budget, although it still usually takes a professional to do it correctly.