BinauralBrisbane, maybe I should try to explain how I work this out when it's my problem. The mikes and preamp weren't specified in your original post, but their specifics have a lot to do with this, which I'll get to (I hope).
For me the #1 constraint is overload. With some equipment a mild overload may sound OK, but I prefer to treat overload as something to avoid absolutely. So one of the first things I want to know about a preamp or recorder is, at what signal voltage do its inputs overload? The key fact here is that the inputs of nearly all preamps and recorders have a maximum voltage that they can handle irrespective of their control settings. If you send in any higher voltage, the inputs will overload and distortion will occur, even if you have the gain controls turned down to the very lowest setting that passes a signal. This is because there almost always is at least one stage of active electronics that the signals have to pass through before they reach the gain control--and that circuitry always has some finite overload limit. The gain control can't help because of where it sits in the overall circuit. The only way to avoid this would be to put the gain control--a passive device--directly at the inputs. For technical reasons this approach is less than optimal from the standpoint of noise and sometimes frequency response, so it is hardly ever used.
My microphones can easily handle the highest sound pressure levels that ever occur in the kind of recording I do. The inputs of my preamps can handle the signal levels which my microphones would put out if their maximum sound pressure level was reached. So with the equipment choices I've made, I don't ever have to worry about preamp input overload. I recommend that people consider that issue when choosing microphones, preamps and/or recorders. The worst case occurs when people try to use professional condenser microphones with the microphone inputs of consumer recording equipment. Those inputs are generally designed for use with dynamic or a certain class of (mostly Asian-made) electret condenser microphones, while professional condenser microphones have typical output voltages ten or more times higher (and many people here record music that gets loud sometimes--I hope I'm not giving away any secrets there!). There are also problems with microphone powering and balanced vs. unbalanced connections.
Those are serious issues, and solving the problems can be quite a headache at times--especially the unbalancing of signals from balanced microphones, which may need to be done differently for each different type of microphone that you use; there's no such thing as a "balanced to unbalanced" adapter that works for all types of microphones, other than a good transformer--but good transformers are expensive and not very small. So a lot of people use mike preamps and feed their outputs into the line inputs of their consumer or "prosumer" recorders, whether that improves the signal-to-noise ratio of their recordings or not; the main thing is, it tidies up the whole set of connection issues and preserves relative sanity in that way.
What about the recorder's line inputs? They probably have an overload limit as well. There's a method that some people use here, which I think makes a good deal of sense: They send in a test signal at a level just below the input overload limit, then turn down the gain control on the recorder so that the meter reads 0 dB (full scale). Then they note where the recorder's gain control is set. That setting is the lowest setting that can be used. If you have to turn the control down any farther to avoid hitting 0 dB on the meter, the input voltage must logically be higher than the input overload point--so you'd be recording distortion even though the meters aren't reaching 0 dB.
While this definitely doesn't answer the whole question, I hope it provides enough to chew on for a while. We can maybe talk about the remaining parts of the question another time.
--best regards