I'm a fairly novice techie, and I'm setting up a rig for a big musical production, with the help of some others, and we are thinking about close micing a guitar amp for the pit band, as opposed to running a line out into the multicore system. Are there any particular advantages or disadvantages to this that we should consider, particularly in terms of sound quality at high volume and practicality with sound control?
The biggest advantage for line out is that you don't have to worry about feedback, and that you can model the tone without worrying about positioning, angles, mics, bleeding, etc; which is a disadvantage of the mic-amp combo, you need to worry about a lot of things to be able to do it correctly.
If you love the sound that comes out of the speakers of the amp, then microphoning the amp has the advantage of capturing the complete tone of the system.
The two deciding factors often are:
Which sound do you like the most, mic-amp or line out?
If you like the mic-amp sound better, will you be able to implement it correctly and successfully? This means no feedbacks, and being able to capture the tone accurately, and making the guitar sound clear in the mix. This also means that you know how to mic an amp: Which mic will you use? In which angle? Pointing to which part of the amp? How close to the amp? You also need to know about feedback, how it's generated, how to avoid it (which also means that you know about PA, monitor, and other sound sources positioning).
The biggest difference between both approaches is the tone. One sounds different than the other. The best guitar amps sound beautiful as a whole (including the speakers) and it's more often than not worth putting a mic on them, but this is not the case for all amps.
So, before anything else, find out which sound you like more with the amps and mics you have available.
Since this has such a big impact in the tone, the guitarist is (normally and ideally) who makes that decision, and if for some reason you can't do what he wants, you do the other option. Seems that you are being able to make that decision, but make sure you take the opinion of the guitarist into consideration. His preference will be reflected in the performance, he will perform better under the conditions he likes the most.
If you liked line out more, just send the out to the mixer (through a DI box and the snake, or directly, depends on your setup).
If you liked the mic sound more, you still have some things to do.
The biggest disadvantage of microphoning an amp live is the danger of generating feedbacks. You need to be very careful with:
Positioning of both the amp and the mic. Both as isolated as you can from other sound sources (PA, monitoring, other amps). Mic pointing to the amp and away from other sound sources.
Mic type. You need a directional mic (cardioid, supercardioid, hypercardioid) that will reject sound that doesn't come from its front (where the amp is).
Check if you can do these. If you can't, go with line out instead. If the mic will be feedbacking, the mic is just not worth it.
Now check if you can implement the amp microphoning correctly. You will check this during the sound check. Now the whole ensemble will be playing, which will test the positioning of your mic and amp. The mic will now capture other stuff.
Can you hear the guitar clearly in the mix? Maybe there is too much going on acoustically around the mic, and it is affecting the tone (even if there is no feedback). Maybe some things are bleeding into the mic too intensely, and you won't be able to add the guitar comfortably to the mix without affecting other elements. Also, it's easier to generate feedback now than ever. You might be able to solve feedback issues here by EQing the frequencies that are giving problems.
Can the guitar be heard in the mix clearly, without feedback issues, and without negatively affecting other elements in the mix? Then go for it! Otherwise, microphoning an amp will have a negative impact in the performance and sound.
It's not a trivial task, but it's worth the try if you really like the sound of the amp-mic combo. You can always just go line out if you can't pull it off, the important thing is to test first.
Line out would remove the influence of the speaker, an important component of the overall tone. Miking would preserve that, with the downside of potential spill from other sources.
A third option would be something like a guitar Pod, many types available these days. That would be capable of giving you speaker emulation at the line outs. The guitarist could use headphones, or you could provide him a separate feed for monitoring.