#1
If I got the Boss ME-20, would I be able to use my Metal Muff for distortion, or would I have to use one of the built in distortions?
My Gear
Schecter C-1 Hellraiser
EHX Metal Muff
Peavey Valvking 112
Boss ME-50
Holy Grail Reverb
#2
I'm guessing you could still use it, just use the clean tones on the ME-20 like you would with just the amp and the Metal Muff.
Main Gear:

ESP Eclipse CTM I
Dean 'Shards' Razorback
Chapman Guitars ML-1
Ibanez AEL40SE Acoustic
Blackstar HT-5 Mini Stack
EHX Metal Muff
EHX Small Clone