I will say this - if the band chases the crowd roar volume trying to be heard, using your backline to do it, all it will do is keep going up collectively... I've experienced that on gigs in ALL genres. As others above have mentioned, it's absolutely CRUCIAL that you stay in touch with venue management and the sound engineer (if there is one) and show that you care about the volume level... and always start out quieter than you think you need to be. It's better to be asked - as long as it's reasonable - to turn UP a little bit, vs turning down, and you will never lose a gig for being too quiet! Again - I speak from experience. If this is indeed a 'restaurant' as much as it's a bar, then being too loud is a death sentence for future rebookings.
It's always worth considering what everyone's collective hearing damage is in the band as well - if people think they are at a reasonable volume but 'reasonable' to them is twice as loud as it needs to be, then the problem will never be solved. I've quit bands before because the 'default volume' of the collective was simply louder than I ever wanted to play at - or listen to as a casual listener. I've worn earplugs since I started playing over 25 years ago... foam ones early on, and then I switched to the Hearos Hi-Fi Professionals with the built-in filters. Honestly, if you are using those plugs and it's still uncomfortably loud, the band is probably destroying the room and everyone in it. Take care of your hearing AND your audience's as much as possible.