i know that there's no solid answer for this question, as setups change with the situation, but is it customary to use a mic-pre to bring the guitars in? if so, would it be to use compression/eq/etc... or should that all be done in the mix later? if youre not gonna use compression/etc, would the mic-pre just be used to color the sound?
You'd normally need a mic-pre to bring the microphone's output up to line level, because the analogue-to-digital convertors in the interface will have better effective bit-depth (=better sound quality) with the highest possible level input. Also, line-level inputs are usually too high an impedance for typical dynamic microphones. These are the only reasons why a mic-pre is necessary as such, and to be worthwhile it needs to use very low-noise circuitry on its front end. A worthwhile mic-pre will also include phantom power which is required for condenser-type microphones, although separate phantom power units are available.
A mic-pre will also colour the sound to some extent. Sometimes good sometimes not. IMO a balance must be struck between achieving a desired colour in the sound and not introducing unacceptable degradation in the sound quality. If you can get less noise and better clarity plugging a dynamic mic into, for example, a Boss eq pedal and using the pedal's "level" slider to apply gain to the signal (using a resistor to shunt the pedal's input impedance to a more appropriate value) than you can using the same mic with a mic-pre, it's probably fair to describe that mic-pre as "crap".
Some folks seem to believe that tubes are inherently noisy things, and that a big increase in noise is an unavoidable price to pay for the increased low-order harmonic density. Manufacturers of cheap tube equipment often take advantage of this general misconception and do not bother to minimise noise at any stage in the product. But it's simply not true - the tube in mic-pre will be acting on a line-level signal so a good SNR should be easily achievable, and a well designed triode circuit can be very quiet indeed. If the single tube in your mic-pre is noisier than the clean channel on an average tube guitar amp, it's probably fair to describe that mic-pre as "crap".
Many interfaces have a mic-pre built in, but the quality varies.
An alternative to using the interface's own mic pre might be to use an external mic-pre feeding the interface's line-level input. Or it might not - sometimes the line-in of the interface simply uses a resistive attenuator to increase the input impedance and pad the signal, bringing a line-level input down to mic level, then using the built-in mic-pre to bring it back up to line level. If the pre in the interface sounds fine but you want the sound colour from a different pre, this isn't a big worry. But if you're contemplating a better external mic-pre improve the sound quality, whether this will actually work depends on the design of the interface.
If you're reamping and can apply compression and eq as (and if) required while mixing then applying them externally isn't a disadvantage - if your hardware sounds better than your software plug-ins or you're pushed for CPU. If reamping like that isn't an option though, it's unlikely you'll get the settings right to sit the sound well in the mix, so if you're mixing ITB you'd have to use plugs anyway.
Basically a mic-pre is needed to get the sound into the DAW. Eq, compression, sound colouration, etc can be done inside or outside of the DAW, and the quality of the results depends on the quality of the tools in either case.
