I think I'm getting closer! I used a MuTron model on the band-pass setting with quite a narrow Q at a 35% mix. It was still lacking. Just for shear morbid curiosity I popped on a sub octave at a 50/50 ratio and BOOM, closest I've ever got.
Close enough for rock n roll anyway.
So:
-Jazz bass, bridge pickup, tone on about 50%
-Digitech Drop with the dry+sub octave setting
-Line 6 Pod Go with a real Big Muff in the FX loop, mixed at 60%
-Mutron filter model, band pass setting, narrow Q, low mix
I understand that I'll never recreate the proper tone without a full studio of goodies at my disposal, but it's definitely close enough to fool the casual observer.