I had not realised you were up against it. Other stuff like bands overplaying and being too loud. Both bad for a good clear monitor sound.
 
One of the best things you can do is to make plain ol stereo recordings of the whole song performance/ rehearsal. PA on in the room everything.  Have a set of monitors in there so you can give instant feedback to
the band. Once they hear it for themselves it should have the desired effect of playing the music better and more simply in a fashion. Softer volumes usually sound better anyway.
 
People playing in between each other instead of over the top of each other is what great bands strive for and achieve. This alone will lower the volume in the room overall a lot yet the monitor will become louder and clearer.
Playing back recordings speeds up the process of working towards making the music sound better.