Run the preamp into the line-input. Set the line input level to about 95, any lower won't help with a hot signal- your levels will look lower on the display, but you will be overloading the input stage. At level 95 at least the peak indicator will indicate overload properly. If too hot, turn down the external preamp or use attenuators. You can add gain with the external preamp without a problem if you need to.
Run the other pair of mics into the mic-input, with mic power turned on. Mic input level 67 (menu set to low gain) is about the same level as line-in level 100. I wouldn't go below something like mic input level 62 for the same reason mentioned above, but you can add as much gain as neccessary for good levels by raising the input level as required without fear.
How much gain you need depends on the sensitivity of the mics and the SPL of the music so I can't help there. If the mics have the same sensitivity (I dunno if they do) then the omnis probably output a somewhat higher level signal since the're more sensitive in the lower bass and produce more ouput down there. So if it's loud and you have the recorder set to those minimum gains already (line=95, mic=62) and are peaking on mic input with the omnis, you might swap them with the card to put the omnis through the preamp>line-in chain if the preamp lets you attenuate gain as well as add it (I'm not sure what the minimum gain on the 9100 is).
There are a few variables, you might try it both ways and see which works better, jsut keep the gains on the recorder at line=95/mic=62(set to low gain) or above.
Hope that helps.