I have approached this from many angles with varying degrees of success. One approach that can actually work IF the musicians will actually do it, is to rearrage their amps to play to themselves instead of the room.
If the amps are pointed AT the musicians and away from the crowd, multiple problems can be solved at the same time. Since the amps are pointed at the musician and therefore the instrument, the volume required for sustain becomes much lower. Guitarists are also well served to learn to get the tube distortion from earlier stages in the amp instead of from the output tubes.
There are ways of successfully addressing the situation. However, the musicians have to get past the feeling that Mom is telling them to turn down and learn to trust to sound engineer, if you are lucky enough to have one that knows the job.
Getting everyone on the stage to hear themselves is a big part of the story, but they also have to learn to trust the sound engineer and not listen to the voices in the crowd during breaks.
Everything starts from some degree of balance on the stage and proper mic selection and placement. With open mics all over the stage there is only so much you can do to bring up someone that is buried in the mix. Everything on the stage is within "hearing" distance of the open mics.
Even bringing up the monitors to compensate for being too loud presents a number of problems that potentially get worse with more open mics on the stage.
One method that sometimes works is to turn up the offending player in the monitors (assuming you have good enough monitors). However in some cases this can actually make the problem worse if they do not react by turning down.
You can use a practice size amp 30 watt or so and get good tone without super lound volumes.
Ti;lt the amp or speaker cab on a 2x4 and aim it at his head and he will turn it down. By the way he can not mix standing in front of a amp.
If you want to try something simple for practice have the guitar amp and bass amp aimed the center of the room as far back as possible have drums on one end vocalist on the other. Guitar and bass player in the middle opposite from their amps. Everyone facing each other so you hear the same thing.
Just some thoughts to encourage trying new ways of getting everyone to the same place.
The volume wars have become so painful I am thinking about bailing- or at least stating my case one final time. The guitar player plays dumb and passive-aggressive- actually gets louder when I complain the volume is not only painful but making it difficult for me to connect to the music. I have a strong singing voice, too- but even wearing headphones the volume is way overkill in the small room we play in. I am indeed being reacted to as the "mom" in the world. Both of us vocalists and the bass player all wear earplugs, but no one complains about the volume but me. I too like it loud- we're a rock 'n roil band- but when we play a gig we are sometimes immediately asked to turn down. We could not get away playing at this volume in most venues, and stadiums are surely not yet in the cards for us. Wearing earplugs is not only not very effective, they're as annoying as condoms. Now my wife insists I get my ears checked because I have trouble hearing her. So our guiotar player is not only annoying the hell out of me but I may be experiencing hearing loss as a result!
A lot of powerd speakers now have a minimal mixer built into them. Check out a 12" JBL eon online and see if that would work. they're not awfully expensive and do a decent job for what they are.
I'd rented the 250, which worked great, then got a great price ($549) from the dealer for a brand-new 300.
At first gig with it, I had to involuntarily throw a temper tantrum (and feel like a fool) because the drummer and guitarist were too loud. We'd hired a sound guy- and simply said- "tell them to turn down." Which they did- reluctantly. The guitarist feels he sucked because he never got the tone he wanted. The vocals are still drowned out on the recordings, but the gig was a success and since then he tries to turn down, though it always seems to fluctuate.
In any case- I strongly recommend the Fender Passport 300. Way more power than I'd expected. And it's absolutely beautiful at home with just me and an acoustic. You can also transport it with one hand. It's like one of those BOSE radios- you look at it and go "nah," then this big sound comes out.
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But the volume issue really needs to be addressed. One of the best ways to remove yourself from a venue's rotation is to play too loud. You're not an asset if people leave due to your volume. Our first comment after our sound check is, "how's the volume...too loud...not loud enough?" If it's too loud, we make adjustments. Anybody can play loud. It takes talent to be able to get your sound at a comfortable level.