Sitting at FOH, you watch the bassist turn on their bass amp. It's pointed right at you. You hear every thumping note that's played. There is no way you're getting them to turn it down. What do you do?
The first thing you have to take into consideration is the direction of the amp on stage. An amp pointed directly at you means that you're going to base your channel volumes around the amp volume. You're also going to base your mix around that amp sound.
The problem with basing volume/mix on the amp is that your ears are in the minority. Most of the rest of the people in the room probably aren't directly in the sound path.
Resolve the directional amp issue by having the bassist point the amp off-axis to you. This can be done either left or right or tilting the amp up. Now you're basing your volumes/mix off of what the majority of people are hearing.
The second option is using the amp as a stage monitor. Some amp's have the capability to run a signal to the house as well as be used as a monitor. If this is the case, have the bassist place the amp at an angle to their head. Using this method, they hear what they need to hear and you have the majority of control over the house sound. Yeah, I know they probably still want to crank their amp – but a short lesson in stage volume and how it affects other musicians might do the trick.
Finally, remember that your words speak volumes to the musicians. Telling them their volume is too loud is pointless – and they can easily counter with "I don't think so!" It only builds that us-versus-them mentality. Therefore, use phrases like "I can't get the best sound from the house mix unless we change your amp setup. But you'll still hear what you need!"
Please note the size of the room can impact how you work with any stage amp. Also, the above also works for guitar amps.
Question: How Do You Deal With Stage Amps?