Is the SA amp driving the TR-2 subwoofer? If so, I'd lean toward connecting it to the 990's subwoofer pre-amp output and letting the 7700 drive all seven full-range speakers. In that case, I'd set the SA's crossover to 200hz (as high as possible, basically) since the 990 will take care of the bass management now, or it appears that you could switch the rear panel control from "low pass" to "full range" and bypass the SA's crossover entirely. You could also set the amp to "mono" (since you only have one sub to drive), but I'm not sure which input it uses in that case (left or right) - might take a little tinkering since the manual isn't clear on this subject. If you are using the SA for something else, you can probably disregard what I just said - let us know what you intend for it to drive and we can go from there.

You could either try splitting the 990's trigger output or use the trigger 2 to turn on the SA. In the former case, you will need to make sure the split doesn't drop the trigger signal too much and cause the amps to not come on. In the latter case, you'll need to make sure to change the "Trigger 2" setting for each input from "OFF" to "ON" so that the trigger gets a signal with every input that you use (unless there are inputs where you don't want the sub active, which I doubt is the case).