The two trim potentiometers are primarily for balancing the two channels. With the trims turned fully clockwise (which is their default setting if you've never adjusted them), the gain through the preamp equals whatever the rotary switch is set to. As you turn each trim counter-clockwise, gain is reduced from the setting of the rotary switch in that channel. Turned fully counter-clockwise, the gain attenuation is infinite (the channel mutes). So in addition to allowing fine-adjustment of gain balance for two microphones with slightly different sensitivities, you can also use them to reduce the gain through both channels by an equal amount. The reduction applied via the trimmers applies to all positions of the rotary gain switch. When the trims are adjusted together - turned down by the same relative amount - they reduce the gain through the preamp below the nominal settings of the main rotary gain switch- from full gain of that switch position (fully clockwise), down to mute (fully counter-clockwise).
It is best to to have the trims set closer to the middle of their range or higher if possible, and not too far counter-clockwise.
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Most small recorders have a certain point on their input level adjustment which you should always remain above, when adjusting input gain to achieve good recording levels. At input level settings lower than that (for louder material) the input stage of the recorder overloads and distort before the signal gets to the metering point. When that happens the meters on the recorder will not show any overload, and often the clip indicator light, if the recorder provides one, will not indicate clipping, however the signal is already distorted by the time it has reached the meters.
That's referred to around here as "brickwalling" because the waveform flattens out at that signal level when viewed in a audio editor, as if it had hit a brick-wall. It is avoided by never setting the gain on the recorder lower than that "don't go below" input gain setting. If you need to set the input gain lower and cannot do so some other way such as reducing gain through the preamp, then you need to use an attenuator between the preamp and recorder which will allow you to set the recorder's input levels above the "don't go below" point without clipping.