Do you have a recorder, like a Zoom or something? Or even a half-decent mic and an interface into a laptop?
I would suggest recording yourselves during practice from a spot out in the room where somebody might be when they listen during a gig. Set up like you're on stage. Get the drummer to play at a reasonable volume and have everybody else play at whatever level they feel they should be playing. Do a PA mix like it's a gig, with the vocals set at a reasonable volume relative to the drums.
The problems should be SUPER OBVIOUS. If you do this in a practice situation, you can play, record, tweak, play, record, tweak until you get things set somewhere that everybody in the band can agree you sound halfway decent. You won't need to be negative or call anybody out, everybody will be able to hear the bad mix.
Alternately if the guitars think they sound great when they're a muddy mess, just make it an "experiment" to have everybody turn waaaay down just for grins and turn them back up in the PA mix. That'll get you an A/B recording that should be obvious which way is better.