Does a REL subwoofer make the speakers' job easier?

speakers get full range signal with REL...JL is maybe what you are looking for...I prefer REL...crossover that doesn't degrade signal to main speakers is often quite expensive, though JL makes a great one...

I was looking at both REL and JL Audio but ended up with Rythmik. I liked the fast servo sub and connect low level from my amp to the sub. My Parasound A21 has an RCA connection for both Left and Right channel and variable gain adjustment and connects directly to each stereo subwoofer(s). I push full range to my speakers and it seems to be easier to blend the low/mid low freqs between the sub and speaker.

I have owned both JL and REL. With JL, the integration was a nightmare (I did not try the external crossover). With REL, integration was seamless without compromising full range signal to my main speakers. The low bass extension with REL is just right (actually amazing), neither overpowering nor underwhelming in my system.  I can’t imagine my system without a pair of REL’s. For two channel system, always connect REL’s with High Level Neutrik Speakon connection, fine tune and enjoy! 

You're thinking too deeply about it.

This is audiophoolery-buy one and try it. There are plenty of happy REL users. The one that aren't simply didn't hear what they were expecting, and went with another brand.

You're not thinking too deeply. You're asking good questions. I had some of these, too. Have both REL and Rythmik. Like both, but Rythmik's adjustable phase is critical to getting room dialed in.