Ooh... I see the confusion, and I'm just now starting to get a handle on what Gallo's doing here. The SA amp is driving a separate low frequency voice coil in the main speakers, but the 7700 is powering the other drivers in the speakers (since the SA amp only supports that secondary voice coil).

If you connect the 7700 to the 990 with balanced interconnects, then you can use the unbalanced left and right outputs from the 990 to feed the SA amp, at which point you'd probably leave the settings on the SA where you had them before (probably around 40Hz for the crossover, stereo mode, and whatever phase settings worked before). You may need to adjust the SA's volume since the 990's balanced outputs will be a few dB hotter than the unbalanced outputs and you'll need to get things integrated between the secondary voice coil and the rest of the speaker because of the amp change anyway. You'll also want to set the front speakers to "large" in the 990, otherwise the SA won't get any signal low enough to make use of.

if you used the unbalanced pre-amp outputs already for the 7700, then you'll probably want to split the pre-amp outputs with a Y-adapter and connect to both the 7700 and the SA. Using balanced for the 7700 will be easier, though.