When coming out of a mic preamp, the signal is no longer at mocrophone levels, it has been amplified to line level. Hence the term "preamp"; a mic preamp's sole purpose in life is to pre-amplify the signal to bring the microphone's signal levels in line with everything else before going to a recording device or speaker amplifier or whatever else you have downstream.
Therefore you would run the output from the mic pre into the "Line In" and not the "Mic In" on the channel strip of the mixer. By doing that you are bypassing the built-in mic pre on the mixer, which you no loner want or need. You then set the levels on the channel strip the same way as you normlly would; set the fader at 0 (a.k.a. "unity gain") and adjust the trim accordingly. You want the maximum signal without clipping or distorting coming out of the mic pre and into the mixer; you should not have to turn the trim high, and in fact may have to keep the trim turned down if the output from your pre is strong enough.
Personally, I'd be tempted to bypass the mixer altogether and chain the preamps directly to the ADATs. Keep the signal path as short and as clean as possible. But either way will work.
G.