you need something like this to connect the amp to the sub via the speaker terminals. Some subs have these build in. some do not.
https://www.crutchfield.com/S-LrFPSJ7FonT/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Ad...
I don't think you can drive a passive sub through the amp from the active sub but not sure. I'd just get two SVS or Rythmik sub and be done with it.
Good luck , fun project!
https://www.crutchfield.com/S-LrFPSJ7FonT/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Ad...
I don't think you can drive a passive sub through the amp from the active sub but not sure. I'd just get two SVS or Rythmik sub and be done with it.
Good luck , fun project!