I have a single REL T5i accompanying a pair of Janszen P8's.
Integration was pretty easy.
The T5i (8" down-firing) is about the smallest sub I could use and still make a difference. With room-coupling, it's enough to level out the frequency response from about 10hz to 35hz. I used REW (frequency response analyzer) to help find a good location.
I tried a much larger , more expensive REL sub but returned it. I found it a little overwhelming. It did not seem to make the system sound better overall and was too heavy to move around easily by myself. Still, I'd like to try one of the smaller Rhythmik subs, just to see if something larger could pressurize the room better without adding distortion.