i am generally in agreeance with the rest of the group. right now i have Phantom Ring->High 5->Octave Multiplexer (i.e. three flavors of octavey channel with a boost) at the front of the chain, then the fuzz array, then an RV-3 and then the filter and LFO based effects. i have a Velcroar at the end of the chain so i also have the option of fuzz after filtering, but generally fuzz after anything else tends to turn the dynamics inside out and mess with the sonics so much that the filtering doesn't act much the same.
you know the theory: octavers track better with the cleanest signal, and filters are more fun with extra harmonics from the fuzz. i sometimes like to run the Phantom Ring into the Octave Multiplexer just to confuse it and make noise. an occasional double stop with it set for faux bass will make it gurgle like a sea monster from a Japanese monster movie.
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FIFTY YEARS OF SCARING THE CHILDREN 1970-2020--and i'm not done yet
How has nobody replied fuzz > filter > filter > fuzz yet?
Different filters going into each other, set to peak at different rates, can sound AWESOME if you're playing long drawn-out stuff. Maybe use a fuzz with a clean blend at the end of the chain so you can get a little bit of the other sounds as well as the double fuzz'd double filter'd... could be pretty stupendous.
I have fuzz>fuzz>pitchshifter>freeze>digidelay>filter (Qtron). That started because my ABSynth wouldn't oscillate right after the QT, so I put the fuzzhuggers in front. I like the QTron as a slow modulation or the growly synth sounds that you get shooting fuzz into the filter. Fuzz is fine before digital pitchshifters but can be wonky before analog octave downs. I like wonky though. Wonk away, all!