I've bought the t-shirt on this type of issue, and I'm gonna give you my 100%. Sorry if I seem to rant or tell folks what they already know. I love to think this will help some suffer less for less time than I did. In no particular order, I offer the following:
The speaker has a cone of sound coming from it. You want to be in the cone. Position the cabinet in such a way that your ears are as centered as can be in the sweet spot of that cone.
Stand and tilt? Hooray.
Keyboard, piano, synth. First piano. Many piano players play in a band context like they're still in piano class. That is, they play full chords with the left hand, and well below middle C. This is your territory, not theirs, in a band context. The piano needs to think keyboards, play at or above middle C, and play thirds and fifths. Of course, I'm generalizing. You get the point.
Synth can be a nightmare if they play a gigantic bass register pedal E or some other obliterating tsunami. They should think about a unique part of the register, with a unique color and a unique part.
Drums. The kick and floor toms can bash the bass out of the bass register to an amazing degree. Consider tuning the kick to not compete. Some drummers don't know how to play with a bass in any way other than locking into a tempo. The part and the tuning make a difference.
EQ. Consider some bass cut. Counter-intuitive perhaps, but it can help a lot. You are wanting to cut sub-bass if you can get to it on your amp, head or board. If not, just take the lows or whatever the bottom third of the EQ is and cut it a bit, then boost the treble a bit. Overall, more bass boost or sometimes even a flat bass part of the EQ is a problem. It just contributes to "boomy".
Settings during practice and full volume. Don't be a sucker for super-clear bass being achieved without the drummer and keys guys bashing away. Wait until the nightmare can happen and THEN see how it all sounds.
Hope this helps a bit.