The sub's signal is assembled from several sources. First, there's the LFE track - if the source has one. Then each small channel is divided into two signals at its chosen crossover point and the low frequency portions of each is sent to the sub.

Phase setting is important, but if the adjustment is purely 0 or 180 degrees you can do the same thing with the 7500 by reversing the positive and negative leads of the speaker cable at the amp.