If the Prosub has a passive crossover builtin, the Hegel amp would deliver power to the crossover in the sub, the low side would feed the sub, and the high side would then feed your speakers. My very first sub 25 years ago (some $80 Radioshack unit) did the same. The Hegel's amplified output will be powering both the sub and the speakers.
As stated earlier, I'd get a nicer sub and run it from the Hegel preamp outs, set the crossover for the sub as best as possible and run the mains full range. It is not the optimal setup, but not much in life is. Optimal will probably involve getting rid of the Hegel, which it sounds like you don't want to do (understandable). If you ever feel the need to upgrade or change the system, having a good sub that can grow with the system makes more sense to me than having to replace everything.