I tried to understand what should be relatively straight forward connections in HT only to get frustrated and lost, repeatedly I might add. I must have assembled 100s of stereo set ups, even with tape loops back in the day, with no problem.
I really don't know why it is that the manufacturers of these pre/pro things wants to keep confusing to the general public but they manage to baffle all but the best of us.
You need a preamp with 2 outputs for subs alone, which you can hook up as left and right stereo channels to the subs amplifiers. I guess you might have better results on a very dedicated HT forum elsewhere. Perhaps even begging the manufacturers to tell you who has that feature if they don't.
I am as I explained a 2 channel fuddy duddy who doesn't usually like even the addition of 1 sub for music applications. In HT it can be a lot of fun.