Let's look at your whole speaker array:

Mains:
46Hz-20kHz (+/- 2dB), 33Hz @ -3dB
Center:
70Hz-20kHz (+/- 2dB), 50Hz @ -3dB
Surrounds:
57Hz-20kHz (+/- 2dB), 43Hz @ -3dB

So you don't want to cross over much lower than probably 70Hz because of your center. I'd probably start out with the crossover at 80Hz. You might also experiment with setting the Monitor 9's to large and small (especially for two-channel music) to see which you prefer.

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