I'll second the FZ-2 suggestion. I tried replacing mine with a several different boutique superfuzz type pedals, assuming a new pricey adaption of the superfuzz circuit should be even better than the already awesome cheap old boss fz-2.
None of em was a gnarly and menacing as the fz-2. It trades in a bit of the vintage fy-2 octave effect and sputter for a bit of gain. 'Gain' probably isn't the right term here, maybe it's the bassy undertones or the more subtle upper octave...

Never messed with a ss/bss buzzz btw, can't comment on that one.
As far as muffs go, my -way too extensive- list of clones and variations I bought and flipped ended with a '74 Maxon d&s. Gainy, and opposed to the majority of big muffs out there the tone control is useable all across the sweep. I'm just not that into the russian muff sound, I had a LSTR for a while, and I really prefer rams head and triangle era muffs.
Another one that stood out, but had too many controls that had little or no purpose to me: Musket.
And lastly: Mastotron. Not a muff as far as I know, or at least it doesn't sound like one. Versatile as shit, especially for all things doomy and heavy. Puts out a ridiculous amount of bass, which I personally really dig.
Honestly though, with the right amp, you can't go wrong with some run-of-the-mill bmp and an ocd or rat in front of it

What amp are you using btw?