REL Subwoofer. Your input requested

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.

I have a pair of REL S5/SHOs with aerial acoustics 7T for stereo. I use this as part of HT with 7.2.2 configuration too and have seen amazing results with both set ups.
I use high level inputs "only" for 2 channel stereo (no LFE) and use both high level and LFEs with HT. This is how REL recommends. 
Agree that new S5/SHO replacement, the S812 have slightly better upgrades, and might be worthwhile to consider. I am going to stick with my S5/SHOs for a while.