REL Subwoofer. Your input requested

Does anyone have feedback on the REL 212?   Specifically what is your opinion of the sub being connected with the speakon wiring via your amplifier vs a balanced output or such via sub out on processor?  Trying to decide if the sales philosophy on connecting your sub on the speaker out via amp is truly better.  I listen to home theater and 2 channel about 50/50 and SACD.   Any thoughts are appreciated.  FYI.  Considering the F212 vs this REL 212.
If your goal is bass that is articulate, tight, fast, powerful, and deep, then the number of subs is far more important than which sub. No one sub can match four reasonably good ones. Nothing else with subs is anywhere near as valuable as if you can learn that one basic truth.

Now as for how its connected, does not matter. If the sub has speaker level inputs that are convenient then use those. Or if the sub has line level inputs that are more convenient then use those. Got it? Nothing matters less than which one you use. Well, except for balanced, which will matter only in the sense it gets you to throw away money on something that's an absolute total waste when it could be spent buying something that's not quite a total waste.

Save your money to buy more subs. Thank me later.
I have the REL S/5 SHO and it's terrific, as is the REL connection system, though I did get the REL upgrade cable...

The High Level Input on REL subs is not a marketing hype or a works amazingly and provides seamless integration of REL subs with your main speakers.

You didn’t provide your room dimensions so it’s hard to recommend one 212/SE or a pair of say S/510’s. From my experience, two subs are better than one. But then again, depending on your room size you may be able to pressurize your room with one 212/SE.

I am using a pair of REL Carbon Limited’s and couldn’t be any happier. I switched from a single JL Audio f212v2 to REL’s.
I have 2 RELs of the same older vintage (a Q150e 10" and a Q108MKII 8"), both bought used (200 bucks each), and although maybe 10 years old or something (not sure) they work perfectly. The "high level" input is the one to use as it replicates the tone of your power amp, although at lower frequencies that may not matter much. The very high impedance applied to the amp means they're basically invisible to the amp relative to their drain on the speaker signal. The single Q150e worked really well for a couple of years, and when I bought the Q108II things worked even better as multiple subs playing the same signal help each other to conquer standing waves (depending of your room of course), and generally just make things work more effortlessly. Great little woofers.
@strenckr i currently have twin JL E-112, which are incredible for home theater/theatrics; explosions, super low bass tracks, rendering Jurassic Park dinosaurs into super huge menacing monsters, Transformer battle scenes into ballistic aural onslaughts (and me wondering if the drywall will come of the studs); the JL control at very low frequencies is astonishing-- think extreme tactile precision. When I remove these 75 pound bass cubes of shock and awe from the HT area to try in the purely two channel things are not as amazing as they were with movies; A bit harder to integrate, lots of twiddling, etc; I've tuned the timing excursion of the JL woofer within 500 uS of the leading edge of the mains woofer; I have found this to be the best for attack and cohesion. Tons of placement experiments; But, I find the overall life of the music is better served with them out than in, unfortunately;
They somehow detract from nuance and musicality. Maybe I just have not adjusted perfectly or maybe I needed 2,3,4 more 12 inch subs..who knows, but it's been many weeks of fussing and i'm done; For now they are back in the HT and things are as good as I could hope for;

For the 2 channel:
...i've been hearing feedback from dealers and forums that REL is the way to go for purely 2 channel. They are very fast, have excellent tone, are easy to integrate, are basically just more musical; They were designed for music first and foremost, apparently; 

I am also looking at the 212/SE; massive sub with 2 active and 2 passive in an all sealed giant cabinet. If you get yours please report back;

I've listened to both the high-level and low-level inputs. In my system I prefer the low-level inputs. 
JL audio makes a great sub. You want to connect it balanced only and you want to use the high pass filter on your main amps. You need at least two subs. Only get one if you plan on getting another in the near future. I would wait and save so you get them at the same time as they are more likely to be matched. 
I've had dual JL Audio F110 v2 with the JL Audio CR-1 and a single JL Audio F112 v2. Now I have dual REL Carbon limited subs. To my ears, in my system,  I prefer REL for 2-channel. I prefer JL Audio for HT.
I have the REL S5 SHO and it it superb on both HT and 2 channel in my medium size room. The RELs have a delicacy and speed that is unmatched by most accounts. They meld seamlessly with main speakers and work together with them recreate an enhanced sense of space which is their strength. They do loud too, but there are better thumpers out there.
Also have a pair of 212SE subs. Haven't quite dialed them in but getting closer. Have speakers with down firing subs in their base so in essence have 4 subs. The RELs have increased my enjoyment of my system. Can't compare them as they are my first subs, but don't have any desire to look at others. 
I had a T9i and now an S3 SHO and Rels are unique in their ease of integration and their finesse.  They are definitely a Sub bass system though.  
My only issue with both of the models I had, when using the Speakon connection, their is not enough control over volume level.  About 6 clicks from zero, there is not enough bass, 7 clicks and there is too much.  This is my only issue with Rels and it really is probably more of a mismatch between the model of Rel I was / am using with certain speakers.  For instance, I should be using 2 T5i or T7i subs with my Arro’s , not an S3....

I bought my S3 SHO as a floor model, no cables, missing grill for $1050 from Magnolia.  Came with warranty though!  They still have an S5 SHO for $1200 (no cables or grill) PM me if anyone wants it!
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 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.