Does a REL subwoofer make the speakers' job easier?

I'm with @bdp24  for sure:

So the Rythmik phase control allows one to position the sub where one wants it in the room (so as to avoid locations where room modes exist---peaks or nulls, a consequence of room dimensions), the phase control then used to align the sub with the main speakers. I would not own a sub without a continuously-variable phase control. The 0/180 phase switch is a joke, far too crude to be of much value in a high performance hi-fi system. IMO.

I did extensive measurement in my room with 3 subs to get the bass within 4 db from 20 to 400 hz. The adjustable phase was indispensable to doing this.

Thank you, bdp24, for the best brief subwoofer tutorial I've ever read. I made a screen shot of it for my now extensive file. It's humbling how much some of you know, and really understand, about the technical side of this hobby.

FWIW, I've got an NHT SubOne in my system, and it has the high-pass filter option attributed here to the Vandy, Hsu, and other designs. I find that connecting with this option does indeed improve the sound of the system as a whole—I presume for the reasons bdp24 so clearly articulated. However, the NHT also has a two-position phase switch, and I've found that engaging this (so, 180 degrees out of phase) also helps very slightly: it improves the integration, so that the overall sound calls no attention whatsoever to the sub while enriching the presentation in subtle but important ways. Perhaps I just happen to have the sub placed where "16 feet" of electronically simulated distance is just right. Or maybe I'm just not that critical a listener.

+1 @bdp24 

Thank you for taking the time to write a response, everyone.  As someone who has never used a sub before, I am really grateful. I am pretty much set to buy a pair of REL subwoofers (picking them up in a few days) and will keep in mind the information you gave me here. Thanks again!

In answer to the original question, a sub can make the main speaker job easier if it is crossed over with high and low pass filters (full crossover). This way the mains are not receiving bass frequencies and can deliver cleaner upper bass and midrange, depending on the main speaker configuration (2way, 3way)..

But the issue of phase and delay has been mentioned. Phase is not really proper time alignment, it is really just putting a late signal in phase, in fact usually making it even later.

Also be aware every sub probably has a few ms latency due to the electronics, cabinet design and impulse response of a large driver). I gather manufacturers are not eager to share this info. To compensate, it is necessary to delay the mains, not the sub. I have done this. One of my subs has 12ms latency, but it is still very nimble responsiveness (it is not sluggish when time aligned correctly)

In my system, there was quite noticeable improvement from even a 6ms adjustment, 12ms is obvious to me and frankly messes with the music timing let alone the acoustic interactions/cancellations.

With all this in mind, the approach promoted by REL to use high level inputs seems fundamentally flawed. Be wary of the marketing the REL dealers use.