When you first learn how to use a mixing console, you will be shown how to set the gain. Your instructor - either in audio school or in the workplace - will emphasize the importance of this and kick your ass every time you get it wrong, until you can't do anything other than get it right.
Setting the gain correctly optimizes the signal level for further processing in the mixing console. EQ, inserts, auxiliary sends, routing, fading, solo and mixing all depend on having the right signal level for optimum performance, and the gain control is precisely where you set that.
But outboard microphone preamplifiers commonly only have rudimentary metering facilities - perhaps only a single 'clip' LED - or no metering at all! So how can you possibly set the gain correctly?
The answer is that firstly the preamplifier needs to be correctly designed with plenty of output level available. For example, the Manley Labs Mono and Dual Mono microphone preamplifiers can supply up to 30 dBV of output before clipping. That is one hell of a voltage in audio terms - around 30 volts, compared to normal operating level of around one volt.
It's probably fair to say that there is no way you are ever going to clip this brute of a preamp, whatever you are driving. So if there is no possibility of clipping, there is no need for metering.
Where you do need metering is in the equipment you are driving with the preamp, probably your recording system. So the procedure for setting the gain with the Manley preamp is to choose the lowest setting of gain, and also set the 'input attenuate' control to its lowest setting (the attenuator controls the level of signal going into the preamp circuit). Then reduce the amount of attenuation while watching the meter on your recorder. When you are getting a good, healthy signal level with a reasonable margin before clipping on your recording, you have the right settings.
If if the level from the mic is too low to reach a good indication on your recorder's meter, increase the gain as far as necessary but no further.
Setting the attenuator and gain controls in this way will optimize the noise and distortion performance of the preamp.
Now since this is a tube preamp with variable 'character' according to the gain setting chosen, then once you have mastered setting the gain in the technically correct way, you are free to break the rules...
When you see smoke coming from your recorder, that's when to stop!