REL Subwoofer. Your input requested

I should also add that in order to try to fix my volume mismatch issue I pulled the Rel out from the corner as much as I could without losing that deep deep bass ( the bass that you almost can’t hear but can feel... 25-30hz?) and also put screwed spikes into the footers to raise the sub.

I know from years of pro sound and home hifi abuses that a long run of unbalanced or RCA (low level inputs on RELs) cables results in funky signal loss and possible gum disease. I do wonder what if any effect there is on the signal from the use of long runs of the speaker output to the "High Level" input utilized by REL. I really have no idea what that might be, but it doesn’t seem to matter much...my longest REL run is maybe 12 feet of my home built Canare Star Quad wires (a very easy build for those considering it, and Speakon plugs are a joy to use), and gets the job done...somehow...

One other option you might want to consider is 2 x S812 SHO.
The 812 is the new upgraded S series that replaces the S5 SHO. It is stack-able with upgraded filters, and improved amplifier and diaphragm among other things.

If I were to upgrade my S5, this is probably the route I would take.
As good as the 212 is, you'll get more placement flexibility and I'd think a bit more performance in the end.

^^^agree with chazzy007.  The new S812 is a worthy successor of S5. I just wish REL bring back wood veneer finishes. Both piano black and white finishes requires bit of upkeep.  

One of the improvements they made with the upgraded S series is an improved filter specifically designed to improve home theater performance.