Because of your center channel, I'm going to give a bit of different advice than Haoleb. Your center is only rated down to 85Hz, while your towers are rated down to 36Hz (+/- 3dB).
I would set the center channel to small and set the crossover to 90Hz, maybe 100Hz. The bass from the center audio channel will be redirected to your front towers (be sure to set them to large and set the sub to off in your receiver's menu).