Volume - as loud as I need to cover the drums on stage.
Mic placement - just off the center of the cone, moving it a bit until it sounds good. (Depending on mic, of course).
Settings - This is a can of worms.
For classic rock, I like channel 2 on vintage, with the gain about 12:00, presence about 9:00, and start at 12:00 on the tones and dial them in, leaving the midrange healthy.
For metalcore, you'll have to ask someone else.