You see pictures of Keith Richard’s guitar amps onstage and there’s a couple or more big Fenders or Mesa Boogies. Then there’s the 2 or 3 thousand watt PA that recieves from the Front of House mixer into which his amps are plugged. In any case there’s the guitar amps and a powerful PA system. I’m wondering are these guitar amps going loud or are they just miked at a modest volume or line in’d to the mixer? If they are going loud then it seems like they would interfere with Keith’s ability to hear the mix sent back to him. So why the big powerful amps?
I’m not sure Keith Richards is doing the same though 
On a big stage (particularly outdoor stages) the sound dissipates rapidly. A 20 watt amp can sound loud when practising at home, but on a big outdoor stage it would be inaudible over the sound of the audience. There is also the visual aspect that a big guitar stack looks “Rock n Roll”.
Small amps can be used on big stages (Chuck Berry generally often to use a small Fender amp) and the sound reinforced trough the wedge monitors at the front of the stage.
That’s the auditory equivalent of crotch stuffing
https://en.wikiquote.org/wiki/The_Simpsons/Season_14#How_I_Spent_My_Strummer_Vacation
Yeah somewhere I read that many bands put up fake Marshall amp fronts to look like they have whole banks of them. LOL!
The short answer is: to get the sound he wants.
– Bill
I’m beginning to see that because an amp has a unique voice but if it’s turned up loud I was thinking it would drown out the mix coming from the monitors thus hindering the players ability to get the whole picture but now I see where Richards and Jagger and others use in-the-ear monitors so I guess in that way they will hear the mixed sound over their amps.
Even with monitor wedges, each performer gets a unique mix. So even if the guitarist is standing in front of a very loud stack the mix in their wedge can be adjusted to they can hear what they need to hear.
– Bill
I saw a youtube video about Metallica’s setup. They have a tech for each guitarist under/back stage who sits in front of a rack unit with their effects/processing etc. He knows the songs inside out and actually stomps on the footpedals to change tones for the band out front. I’ll see if I can find it.
In a lot of cases, they feed the signal from the players processing stack into the PA and let the PA do the work/amplification.
Here it is: https://www.youtube.com/watch?v=TdidWLkd7TI
Damien
After lots of watching and reading it looks like as you say the monitors overpower the individual players amps. That was my main question but I read where on one occasion Keith Richards had his amps turned up to the max. I guess you have to kind of be there to really understand how they make it work. He also often uses two different amps to get his sound which was pointed out in that Metallica video.
Maybe he has a hearing problem. In his autobiography Clapton says he only has maybe 25% of his hearing.