Do you need a second sub….maybe/maybe not.  A lot depends on your room acoustics and the potential for standing sound waves.  Adding a second sub can fix that with parallel walls.  It gets a bit technical and you will have to take some room measurements…so I would do some reading on standing sound waves.  

Using the high level speaker inputs on the sub is a preferred method by most here on the forum.  My setup is a bit different only because of my room setup and the optimal placement for my sub.  In my system I utilize the Decware ZSB switch box which has 4 inputs and 2 pairs of RCA outputs.  When I was building my house, I had run a pair of RCA cables in the wall. One set of the RCA outputs go to my CSP preamp, and the second pair are connected to the sub which is located on a side wall.  

Hope this helps.