I was strongly considering REL as well, and believe they are a good choice.
However, the REL hook-up utilizes the high-level speaker wire for input. So both the main speakers and subwoofer operate from the same signal.
I went instead with a Rythmik F12, which operates from the RCA low level input, then the Rythmik has a HPF (high pass filter) RCA out to feed your power amp, then your mains.
The benefit that it reduces the amount of low frequency requirement going to the mains. This reduces the amount of woofer excursion in the mains, improving their clarity.
I was less familiar with Rythmik at first, but I had earlier purchased a pair of Fritz Carbon 7's. I spoke with Fritz before choosing the sub to pair with them and he recommended a sub with the "servo" technology, which the Rythmik has.