REL Subwoofer. Your input requested

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. 

looks like REL just released the new 212/SX