my basses are really quiet without an amp.
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But seriously folks... for me, the sound of my bass through stage monitors often isn't what I want. I want my instrument to sound LOUDER than the rest of the band, just like everyone does for their own instrument.
When you play your axe, it is louder if you have an amp by you... if it's an acoustic instrument, it's louder because it's next to you.
The disconnect when your axe is sitting perfectly in the mix while performing can be frustrating.
Also, I need to hear myself louder than the other instruments to make sure I catch my own playing side effects, tone side effects, tuning, etc, before it's so bad that it comes through in the general mix heard by everyone else.
Also... most stage monitor mixes aren't unique for the bassist - you can't generally ask for your own custom mix in your monitor. If you could, on those rare cases I've been able to, a bass amp became less important.
BUT, I still prefer an amp - I can move my head to hear more me, less me, how it fits in the mix etc, without asking anyone else, as required. It's just a way to get much more flexibility when performing.