In a whole lot of consumer recording equipment, the microphone and line inputs are connected to the very same circuitry, aside from any microphone powering that may be built in. The only difference is that the gain of the input stage is lower for the line inputs, because a lower degree of negative feedback is set for the circuit whenever a plug is inserted into the mike input. (Lower negative feedback -> higher gain and vice versa.)
So a lot of the time when people here talk about "bypassing" the mike preamps of a recorder, there's no actual bypassing going on; line-level signals go through exactly the same active stages of amplification as if you plugged mikes into the mike inputs. You're just dividing the gain between two components (the outboard preamp and the recorder) instead of having one component (the recorder alone) provide it all.
Of course the outboard preamp may be more suitable in some way (impedance, powering, noise floor, overload limit, frequency response, distortion, freedom from interference, visual sex appeal) than the recorder, or it may simply be more to a person's liking. And there are some recorders that really do have an extra stage for bringing microphone signals up to aux or line levels. I'm just saying that the concept implied by the usual language doesn't correspond to how consumer gear really works a lot of the time. Sorry if this spoils anyone's sense of purism ...
--best regards