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Well well well. I bought all of them for guitar but they are still great with bass too.
EHX by Sovtek Green Russian Big Muff - real classics, great box but it steals highs from you signal being turned off. So you should put it in a loop or make a true-bypass mode. Sounds cool into the most of the bass or guitar amps. But if you are pluging your bass into it - everything depends on your cab, yep it can sound like a crap through some cabinets. You got to dial tone knobs carefully, its easy to some hissy fizzy sound on high freqs.
Univox Super Fuzz - legendary stuff. I can`t say it suits for a lot of music styles, but if you need that sound of a crushing mountain - you will get it with superfuzz. Just don't know what to add
Boss HM-2 - mostly known as the spirit of swedish death metal. But if you roll off "high" a bit - then you'll get that fat and heavy tone. Something between fuzz and distortion. My personal favourite!
P.S. in case if you're interested to read and hear about matamp gt-200 , please welcome :
Matamp GT200 review + pix & vid