If all you want to do is have stereo subs supporting your left and right channels (no LFE, no crossover of data from other channels to the subs), you could simply connect the left and right pre-amp outputs to the ICBM, then connect the Mackies and subs to the ICBM. The only settings that would be relevant on the ICBM would then be the front crossover and the sub controls. In this case (and in any case with the ICBM upsteam of the Mackies), I would recommend bypassing crossover controls at the Mackies.

If you want to have stereo subs that also get LFE data from the .1 subwoofer output of your receiver and low frequency data from the center and surrounds, you will need to connect all six pre-amp channels (left, right, center, both surrounds, and sub) from the receiver to the ICBM, set all speakers to "large" at the receiver, and provide amps downstream of the ICBM for the center and both surrounds.