This is such a ridiculously broad and stupid question that I don't know how to search for existing threads. I know, in the end, it's going to boil down to "Just use yer ears, mate," but a little head start would help.
I don't play at drum volume a lot. Never have. Now I'm jamming covers with these guys, walking cold into practice rooms, plugging in and struggling to sound good. Struggling to be audible without being too loud.
The backline gear is not the issue. Last night it was a Darkglass Microtubes of some sort (into I-don't-know-what-cab); last time it was an Aguilar Tone Hammer. I'm driving a P with a Lindy through a MarkBass Compressore, into the head. I'm moving my fingers from the neck back to the bridge, trying to find a spot where the E string isn't invisible and the D string isn't massive.
The question, I think, is figuring out what room nodes (is that the term?) are doing to the tonal balance and using EQ to counter them. (In retrospect, I think last night I was primarily standing in a horrible place, next to a wall across the room from the cab.) And also, how to find a place to live alongside the two guitar players and keyboard player.
So yeah. How do you do that? You walk into a room, you're getting set up; where do you start with the EQ?