I believe the LS100 has multiple Pre outs, 2 RCA and one XLR (Not a balanced output, just there for convenience). I think all outputs are active all the time.
Run one set of outputs to your LS9 Amps and another set of RCA cables to the plate amps. Check out the Phase issue on the LS100. You will need to turn on the EQ on the Plate Amps and adjust the Phase so that all the drivers (LS9 and Subs) are playing correctly with one another.
You can consider making some inline filters if you want, but probably not needed. Gives some piece of mind for protecting LS9 drivers from the bottom octave if you are playing crazy low synth music or the Frequency sweeps like Tyson has been doing (40 hz to 0 hz - Yikes!)
Other threads on the GR page on how to do inline filters and YouTube video as well.