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Play at appropriate volumes, even when amplified.

John Lee Hooker, when confronted with a very loud room (loud audience), would play very softly, until the audience quieted down. So should you.

Think this is a case of "if it's too loud, you're too old"? Wrong.

Blues music (or really, most roots-based American music) gets its intensity from dynamics and feel. Volume is a fairly recent addition to music, and is mistakenly substituted for feel and dynamics to bring an intensity to the listening experience. Volume is a 'rock thing', it has no place in traditional music.

If you're playing in a stadium, good on ya - but let the sound man handle your volume. you don't have to overwhelm your audience to make your point - finesse them instead. Lull them, swing them, surprise them, make them laugh or cry... but you don't need volume to do any of those things. Once your volume goes to an assaultive level, you have nowhere else to go - you've lost the opportunity to nuance your listeners.

Here's another point: If you're in a bar band playing several sets a night, your volume will have a tendency to creep up. Don't let it happen. Keep the volume under control and you'll have a LOT less fatigue at the end of the night. You'll also be able to hear your bandmates better in the last set!

You only need 'enough' volume. Enough to get your tubes or speaker singing the the way you need them to. Enough to stay in the mix. Enough to be heard in the back of the room.

This is important: If you can't hear yourself, DON'T turn UP - get everyone to TURN DOWN. If you can't do that, find another band. I'm serious here.

You do NOT need (this is important, read it twice if you need to) enough volume to play over your drummer.  

If your drummer cannot moderate his/her volume, 
s/he has no business on stage with a pro band, 
especially a Blues band

Get another drummer. Good musicians (even drummers) play at appropriate volume levels, period. Be a good musician, not the other kind.