How are gentle slope passive EQ (equalizers) created?

At your risk. Using patent US3624298? ie bridged T constant K with varying resistors on each of the inverse circuits, one being inductor, the other capacitor? For example by using equations from Elements on Sound recording - frayne, Wolfe... for a Shure SM58 microphone impedance of 314 ohms, left and right resistor of the T network, 314 Ohms each, lower shunt inductor 100mH, upper series capacitor approx. 1uF. Equations are L/C=R times R, l=R/(PIxfc), C=1/(PIxfcxR). For Fc=1KHz. Or using a simpler passive EQ described in Audio Cyclopedia - Tremain, where inductor circuit is connected to tap of variable resistor, capacitor circuit connected to tap of other variable resistor, each potentiometer for LF, HF bass, treble (attenuate or boost) respectively. If memory is correct potentiometers are in parallel with signal out. Or using just an inductor in series with variable resistor and total in parallel with mic, ie Pleiades (R,L) filter but with component valkues recalculated for a lower cutoff like the Cinema Eng EQ? By the way what is the variable filter, EQ inside the so nice Sennheiser MD421 microphone? By the way the AKG D 330 microphone used by ABBA at live shows? has 2 very nice EQs: bass correct, high increase. The Shure SM7 has a nice bass correction or low cut? filter and a broad band? reject, apparently to make the high frequency peak flat, schematic can be found on www.coutant.org. The AKG D1000 microphone has a nice bass correction passive filter too. Other non insertion loss way could be an inductor with core magnetic permeability that varies with frequency. This would create an inductance that varies in frequency in a different way to 6dB per octave. Some audio transformers (with silicon steel? mumetal?) such as Hammond transformers seem to have this property by measuring the primary inductance. If a low primary inductace transformer is choosen or the mic impedance is steped up by another transformer then the cutoff frequency could be moved to 1KHz which is close to a frequency in the Cinema Enguineering low frequency correcting side of the 4031-B program equalizer. Such inductance is to be connected in parallel with the microphone output.