REL Subwoofer. Your input requested

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;
Cheers!

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.