Well, my advice is to start flat. (It would help to know what bass and amp we are talking about here.) I play passive Fender basses mainly so "flat" is wide open on the tone control. And all my EQ knobs are set to "zero". Now, zero on your amp and zero on my amp could be different. Forget what the numbers on the dials say. Look up what "flat" is for your amp. It could be Bass at 2, Mids at 0 and Treble at 2 (just for instance). Once I get everything flat, I simply play a little. If I need a little more low-mids (and like Floyd said that's what will generally get you really thick but still cutting through in most rooms) I simply boost them a little. If I am too boomy, I cut the lows a bit. (You get the point.)
But also as Floyd and B-string said what you hear on stage may have nothing to do with the way it sounds out front. So be prepared to give a little on "your tone". The ultimate goal is for you to hear what you are playing so you can keep time, and for the audience to hear the tone that best sits in the mix. Sometimes they are the same tone, and sometimes not so much.