Hehe. I had a guitar player who "couldn't hear his monitor" so I setup a pair of JBL MPro 115s for his monitors and drove them with an RMX2450 at just below clip. He would still point down at the monitors, then his ears, indicating that he couldn't hear his vox. I could mute the mains, and his guitar and monitors were so loud, even from my mix position. I finally raised them up off of the floor on some cases to get through the show.
And yes, I kept asking him to turn down his amp, but he insisted that he wouldn't get "his tone" if he lowered the volume.
There is such a big difference with stage volume in bands that work with a full PA regularly and those that aren't used to it.