I’m using two Rel, T/7x with Magnepan 1.7i speakers and they work amazingly well in my large room. I started out with just one, which sounded very good, but adding a second one took the bass to another level. They blend so well with the Maggies, that you can’t tell your listening to subs. They’re fast & tight & keep up with the speed of the Maggies. I personally don't see the need for the Rel S series.
Here are some videos from Paul McGowan of PS Audio, giving his take on connecting subs to amp outputs vs pre-amp outputs. Also, his take on one vs two subs.
The best way to connect a subwoofer - YouTube
Two subwoofers vs one (youtube.com)