A good stage mix is critical to making the rest of the venue sound great. As a band, part of your rehearsal process needs to be learning to blend together. Ultimately the thing you're all getting together to do, is to create A Sound. If the guitar amp is cranking away so the keyboardist can't hear the vocals, then your not working together as a band. All of you ought to be able to hear each other without re-enforcing the amps through the monitors.
I like to advise bands to bring a living room mix to the stage. since most stages aren't even that big. When you get together to rehearse in a living room or similar size practice space. Decide on spending one entire rehearsal session on setting your levels. Use the drums as the standard (since they have no volume knob) and have everyone set their levels to mix well with that. No-one should be louder than the drums. Start with the bass and work out what everyone agrees is a good bass drum mix. Then add the guitars etc. Use your stage monitors for the vocals since that's what you'll be hearing on stage, and keep adjusting everybody's amps till you can all hear everything, and nothing gets drowned out. Again no-one gets louder than the drums.
And don't let the Guitar player get away with claiming he can't turn down anymore. "but I'm not even on 2!" No-one cares what number your on, your too loud! Part of being a professional musician means learning how to use your gear. It also means knowing what gear to get.
There is absolutely no reason to get a 100w amp for a guitar. You will never be able to use that much power, and you'll always be setting your volume to less than 2. you'll get better tone and a much better mix if you use a 25w amp that you can turn up to 7!
If your playing a small gig the amp will work great, and if your playing a big gig, then you'll be mic'ing the amp and the band will still sound great.
Even if you have to use a big amp, now a days you get most of your tone from your pedals. Turn up their inputs so they run warm or hot, and then TURN DOWN their outputs so that the input they send to the amp isn't so hot that your overdriving at 1. If you take the time to set it right, your big amp can work like a power amp giving you honest re-production of your pedal sounds.
Once you've developed a good stage mix, you can take that mix everywhere. In the small clubs your sound is all ready to go. And when you get to the the bigger clubs, the sound guy just takes that great sound you've all worked on and puts it into the mains and whalla, you sound great. Even when you finally get to a really big stage and your all spread apart, you can still keep your levels and ask for some reinforcement in the monitors. A band who brings a good sound to the stage is easy to make sound good.
That being said, I think I need to say a bit about Bluegrass. You need gear too. Really. even if it's just some good Pick ups. You guys need to get over yourselves. Don't get me wrong, I like Bluegrass. I really do. But when you show up at a rock show or in a noisy dance club and ask for 12 mics cause your purists and never use pickups, cause you you think that's how you get that 'acoustic sound'. Well you drive me, and every other sound guy nuts. Especially when you then ask for more of ME in the monitors please.
The reason it's so tough to get enough sound out of a mic'd instrument in a live show is FEEDBACK. if you don't know what causes all that feedback, well it's when the sound that comes out of the speakers gets picked up or heard by the microphone, which then gets amplified out the speakers, which then gets heard even better by the microphone, which then gets amplified even more by the speakers etc. etc. in a quick loop that screams out of the PA much to the discomfort of everyone in the room.
Why doesn't that happen in the studio or at the Grisman Show? Well first, in the studio your mic's aren't in the vicinity of the speakers so there's no chance of starting a feedback loop. and at the Grisman Show, he's usually in a theater, and theater's are really two rooms (sound wise) there's the front of house, and offset from that is the stage. The Speakers are not only isolated from the stage by archatectual design, they are 40 feet away. Less chance of feedback.
A sound guy at a live show is always trying to balance a ratio of Gain before Feedback. It's the art of trying to get all the volume you can out of a microphone but not cross the threshold of feedback. Every sound system has certain frequencies that it reproduces better than others. And every room has certain frequencies that it reflects louder than others. Either of these and especially the combination of the 2 can set the system into a feedback loop. By finding these frequencies and cutting them out you can push the mic up a bit further till you find the next frequency that feeds back and cut it. by continuing on with this you get to the point where all your frequencies are feeding back. at that point you have achieved your maximum volume. Back it off a bit and see how much of your previously cut frequencies you can bring back. Gently slide each of them up till it just starts to ring then back it off till it stops.
If after you've done that and the acoustic guitar player's girlfriend says to turn him up. Tell her to buy him a pickup for his birthday (as long as his birthday is before the next time they'll be returning to this stage). Back in the 50's when Les Paul invented the Electric guitar, it was to solve the Gain before Feedback issue. not to deviate from acoustic sound.
AND if you think after making all those Laborious EQ cuts and shapes that your going to still have that "acoustic sound" your trippin on mountain dew. That sound is only done with a very sensitive mic turned way up in a studio or a big theater stage. NOT in a bar. Or you can get a GOOD pickup. I'm sure you've had experience with a crappy pickup. But now that your leaving your living room or the front porch or wherever else you used to play. YOU NEED PROFESSIONAL GEAR. just like every one else. The rock guys have to spend way more for their amps etc.. We're just asking you to buy a decent pickup for your instrument.
And here is something else for you purists to remember. The moment the sound of your Acoustic instrument enters that microphone, it becomes ELECTRIC. Acoustic only truly happens on your porch when your playing for a few family and friends. Micing an instrument converts the audio process to Electric. Accept it and get a pickup. You'll thank yourself for it, I promise.
In a noisy or feedback prone environment, a Pick-up is the best way to get a strong signal from your instrument. No microphone can compete with a pickup when it comes to gain before feedback.
So please, Get a Pick-up already. If you'd do that, you'd be amazed how good your bluegrass band could sound in a live club.