@kyro By non-linear I was referring to things like [pow~] getting stuck in the feedback loop, not a correct terminology but one I fall back on with synths because it was a common term in the synth diy community 20 years ago to refer to anything that is not the normal distortion/filtering type stuff in a filters feedback loop.
@manuels The magic of the moog (as in minimoog) is primarily in the output amp which affects the entire filter and is why the ARP version sounds so different, they 'fixed' it and lost the characteristic flavor of the mini's filter. The output amp on the mini's filter does not have anywhere high enough of an impedance to deal with the ladder which drags down frequency response and distorts things, feedback loop comes off output amp so nothing in the filter is untouched by the output amps flaws and it is more the heart of the filter than the ladder. Later Moog ladders are closer to the ARP version, output amp has a high enough impedance and we loose most of the mini's character but we get a decent response in the upper registers in trade.
Most of the classic VCFs are just cascaded 6db filters with a feedback loop for resonance and is my standard, the various 'proper' filters created in the digital world are lacking in many ways to my ears. The cascaded nature of many circuits in the analog world is part of what the digital world is missing, in a 4 pole filter that means we at least have input amp, 4 filter stages, output stage and feedback each giving their own distrotion and frequency characteristic. The filter stages we try and balance and get them to cancel out any quirks to work together but they still have an individual character and input, output and feedback have a very strong character which gives us a rather dynamic result when everything is combined.
And since analog feedback is basically delayless (which in digital land is of course impossibe)
Check my thread on feedback which actually came about because of trying to get a decent filter executed in pd, getting closer.