well im gonna go against the grain of this thread so far: growl
in minimoog is something that is inherent in its sound in low mids and lows. it is always there. it does not exclusively describe the feedback via external input (i.e. "headphone out trick").
its a sound character atribute for old moogs and their modulars used for ages. and easily heard. a direct result of the design, the circuits and the components used. without going into detail past of my rudimentary comprehension of electronics, the circuits most to blame are the pre-vcf mixer and levels of oscillators entering, the vcf, the vca and their coupling. vca having peticular (desirable) distortion behaviour coming from matched transistors etc etc.
i dunno why exactly, since this this is not rocket science, but its seems one of most elusive things for manufacturers of modern, contemporary analogs to achieve. all the elements of this have been throrougly studied for ages. maybe it is those old transistors, or noisy totally un-hifi opamps like 741. i mean, original SEM definitely growls too, and is full of 741 and 301 opamps.
and seems whatever praised cherished equipment from 70s you open, you run into these little buggers.. be it synths, rack fx, pedals, outboard..
back on topic, to get somewhat into ballpark, yes you could run into an external ladder filter, but you also need a VCA with accurate sound of mini. some of my friends help themselves by using a Neve type pre they already have in the studio.. and it does but hey... its overkill, and for that money you can already pick up the real thing. you could try something cheaper like golden age pre73 but im not sure if that is any good.
tho all these are half baked solutions. its easier and more cost effective to pickup a smaller moog from late 70s. because, even if you have complete minimoogs back end (vcff+vca) it aint gonna sound correct if you feed it oscillators from a va/vsti.. minimoogs oscillators are unlike anything else...an organic being with a life of its own..
they should teach about em in biology class