As if the dial was a clock. Appreciate your reply. As stated in the OP telling guitarist anything is not an option. Is there anything that I can re: my setup...
Then you have no means to get great sound at all. Or even slightly better. It will only be worse. On any venue. Then you should move on. Period. I have yet to experience this, haven't seen/hear one instance of it as of yet, and I am not expecting to see it anytime soon. And I will not try to wait for it to happen.
You should go for rented PA with a FOH soundman. I even know a soundman who - during the break between soundcheck and gig - pulled two power tubes out of the gtrds Marshall stack, without telling him. It had four. Not every Marshall can take this, but he knew. When the gtrd came back, he cranked the volume, but couldn't tell the diff, as the FOH turned it up louder in the monitors, as he couldn't do if it had all tubes in at 100W. He put it back later on without telling him.
If everybody else has to adapt to the guitarist loudness but it's not mutual, it's time to leave. Although he has the upper hand providing PA and may provide rehearsal space to you all at no or little cost, it's mostly THAT he uses an excuse so he can keep doing it. I wouldn't even join such a band in the first place.
This is how you set volume and great sound of any PA at any gig, especially if you own your own PA and has consistency from gig to gig, especially since you have electronic drums, and VOCALS:
1. Let the lead vocalist sing into the mic. As loud and as soft as possible.
2. Turn it up just so it feeds back and then back off just below feedback level. Set the max volume without feedback.
3. The resulting volume and loudness is the one that should be the yardstick for the rest of the band to adapt to. And make concessions and compromise to. Being able to work mutually with sound levels.
If anyone in the band doesn't comply to this, well, in my book, they have no place in any band at all, really. They can start out as a solo act performing solo on stage, where the only instrument is their own, churning out riffs at whatever level they want. Doesn't even need any PA. And instrumental too, no vocals at all.
I've always wondered this, whenever playing open mic jam, rehearsal space jams, and theres a plethora of different amps. If someone can't play louder, the band members and audience members always go "
we can't hear you, turn it up!". Instead of telling all others to turn it down because one person is already at maximum levels, especially singers. This beats me why there's always people asking ONE person to turn up - regardless of that they can't - instead of asking the rest to turn down so the weakest volume gets a chance to be heard. This beats me. And has been going on for ages without anything really happening. No one has never ever learned anything from this. IME.
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EDIT: And seriously, as others have already said: Ampeg SVT 810 and not getting heard? Is this a put on or what? I don't even want to attend any concert of your band, in spite of it being outdoor festival open field.