I've always used my big muff turned really low, so it gives off a low blues kind of distortion. Recently I had started using my amps distortion instead, when I accidently clicked my big muff on and I suddenly found myself with an onslaught of power coming out of my guitar.

Two distortions were on at this point. I've seen pete townsend do that, jack white too, and I think 'maybe' hendrix, not sure. Anyway I love it, it's a shame it feedbacks so back when I mute the strings, but when I'm playing it's fine. I tried turning my big muff settings really high while my amp was on clean. But it doesn't give off the same effect, it just sounds like what it should, a high amount of fuzz.

I've heard that when you turn the volume and gain up full on a fuzz face you get the same effect as what I've got, even with the amp on clean. I'm going to get one to find out.

How do you use your Big Muff/fuzz?
I cascade my Bad Monkey (Level and bass noon, trebble at 2, gain at 3) into my russian muff (volume 2, gain and tone dimed) into the VJ cranked to noon, and get the biggest distortion I have ever heard. It's usuable too, rather than just disgusting and noisy.
I keep my big muff's level at about 12 o'clock, the sustain at somewhere between 9 and 12 o'clock and the tone varies between 11 and 3 o'clock. I generally use it on the clean channel of my amp.
...Bleep Bloop...
When I'm using 2 distortions together, my big muff (volume and sustain on full) is always first. I'd like to try it the other way around but all I've got is my amps distortion and no effects loop...
sometimes i use it with gain quite low for that bluesy distortion and then i roll back the volume on my guitar for an overdriven sound, then sometimes i have the gain on about 4'oclock for solo sort of stuff, then i turn my DS-2 on at the same time for immense distortion stuff