If you were about in the 70s, then yes - that was what amps were for - channel 1 mic, channel 2 guitar. That's certainly how we thought they should be used - two jacks, two sound sources. Done!
Seriously though. I am one of those sold on the only thing I want from a preamp is gain. No changes other than the amplitude coming out being bigger. We can all live with differing amounts of noise, and once at my working level, I won't pay more money for less noise, because I don't benefit. I certainly don't want the subtle changes and distortion - even if it's third harmonic type that makes even my bad singing and voice better - because it won't be! I never laugh at the people who spend fortunes on this devices - it's their money. They consider it good value, I consider it not. I'd for laziness sake use the wrong mic and then warm it up with EQ. So few mics are able to be used flat - this 'character', I'm happy to simulate with EQ. A mic with a small hump at say 5K - well, if we use EQ to make it flat, or use another mic and add in the same hump - I can usually get close enough. Of course mic frequency responses change with the size of the capsule and housing meaning that any frequency response curve is a point source result. Rotate or tilt the mic slightly and it's a different response - and I think it's the 360 degree sensitivity and frequency response that make some mics different and maybe special - not the on-axis sound but the rest of the capture response. If a certain preamp colours the sound nicely, that is an effect - it is not a sign of quality, quite the opposite.
There was mention of transient issues, but that's a symptom of non-linearity in the design - so an unwanted change. Video camera preamps and their auto systems often convince me that when you switch auto level off, it is still happening - but I suspect it's really just mild compression that cannot be switched off.
