Does a REL subwoofer make the speakers' job easier?

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.

1.  The sub would make the speakers’ job easier by not sending the low bass signal (i.e., below the crossover point) to them.

This can be true as long as the crossover in question has high pass filter capabilities. A feature very few powered subwoofers posses. A couple of exceptions are the JL Audio “E” series and I believe the Audio Kinesis “Swarm” (actually passive subs with an external amp). The other option would be to use an external crossover connected to your preamp outputs, which would split the signal that feeds your main and sub amps. Some examples of these would be the JL Audio CR-1, miniDSP as well as units made by dbx and XTA.

2. The signal sent to the speakers would still include the low bass even when the sub is used. Therefore the speakers would still receive the full range signal.

This is true with the connection method that you present.

Option 1 makes the job of your main speakers easier, option 2 does not.