Yet another Sub Question

If you have one and like it get another, or sure upgrade a level or two. The second extends the soundstage, can make a big difference.

However, don’t overdo the volume. Typically they are set to very low volumes or they overwhelm the system. But REL has great software for setup, so careful setup and you’ll be good.

Yeah I currently have the existing 9x set really low.  Don't take much and mud happens

Your stated main speaker response isn't digging very deep so the sub will have to reach up a bit to cover the mains.  This being the case, I would expect that a two sub setup will blend much better and produce fewer and less aggressive nodes.  If your mains were rock solid down to 35hz one sub covering the bottom octave 20-40hz might get it done but that's not your reality.  It would be ideal to get a dealer demo loaner to play with your existing sub to see how the pair works in your space.  Good luck and cheers