I turn the Gain in the Global Graphic EQ down to -12 and it helps a bunch.
Or turn down level on the front panel....
Or turn down the trim at the console. It's a rare mixer that can't be turned down far enough to handle a line-level signal.
They should be plugging an Axe-FX into one of those inputs with the line/mic buttons. Vocals don't need those, and should be moved down. I use a smaller version of the same thing at home for band practice, only mine is branded "PHONIC" instead of Yamaha.box mixers like this don't have a trim control only level. so if you plug into an XLR, you are forced with a Mic Level connection, and with a line level device, you can barely turn up the Level knob and it's already way too loud.
there are other non-box mixers with this same problem and the not-so-occasional problem of no mixer, plug into the house stereo system.
Vocals don't need those, and should be moved down.
but many people use the Compression (yellow knob) for vocals. regardless, just showing an example where there are actually situations where there is no options (when you need to use an XLR cable for long cable runs).