Does a REL subwoofer make the speakers' job easier?

I've run my main speakers full range with a couple of RELs for years and it sounds great. "Classdstreamer" answers his own question with 'my REL sub does perform super well in its system." Subs don't need to have adjustable "gradation" as things are either in phase or not. REL owners rarely wonder about that. It's simple, and easy to see phase adjustments work with RELs. Good subs add that bit of reality where low frequencies exist (as in the actual world) so main speakers can seem to take on more life-like tone.

Phase can be a somewhat complex matter, but in the case of subs it is not. The phase switch on most subs gives only two options: either the signal is left as received, or it is rotated 180 degrees (the phase is flipped, i.e. reversed).

The phase control included in the Rythmik "standard size" plate amps is not a 0/180 switch, but rather a continuously-variable rotary knob control, providing from 0 degrees of phase rotation (no delay) to 180 degrees of phase rotation (16 milliseconds of delay). In 16ms sound travels approximately 16 feet (one foot per ms), so setting the Rythmik phase knob control to 180 degrees/16ms does the same thing as moving the subwoofer back 16 feet, but does it electronically instead of physically.

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.

As for the REL subs making the speakers’ job easier: as others have already said, the REL (and almost all other dubs) provides no filtering for the loudspeakers, so they and their amp(s) still "see" a full range signal. Not relieving the main amp and loudspeakers of having to reproduce low frequencies is to ignore one of the benefits of using sub(s). Bass frequencies "eat" far more of the power an amp creates than do higher frequencies; if you remove low frequencies from the signal the main amp receives, far more power will be available to the loudspeakers, and it will be lower in distortion. Removing the low bass duties from a loudspeaker will allow it to play louder, and with less distortion, especially true with planars.

As you read in an above post, Vandersteen offers a high-pass filter to remove bass from the signal sent to the main amp when using his subs; HSU does too. But you can easily install a simple 1st-order (6dB/octave) filter on the input jacks of your main amp; it consists of a single capacitor, the value of which is dependent upon the frequency at which you wish to crossover from your speakers to sub, and the input impedance of the amp. The formula to find the correct cap value may be found on the ’net.

Another route to take is to install a separate electronic crossover in your system, in-between your pre-amp and the amp for the speakers. A benefit of using a separate x/o instead of the Vandersteen or single-cap filter is that the x/o frequency is usually adjustable. If all you need is a 1st-order filter, the old Dahlquist will work okay (especially if you replace the original parts with higher quality ones), and can be found used for a coupla hundred bucks. Really good x/o’s provide adjustments for both frequency and slope (6/12/18/24dB/octave, 1st/2nd/3rd/4th-order). Pass makes a great one, but it of course ain’t cheap. Nelson Pass offered a cool little one in his First Watt line (model no. B4. Get it? ;-), but it is no longer available assembled, only as a DIY kit.

By the way, though REL is most well known for offering a high-level sub connection, the non-XLR versions of the Rythmik plate amps do as well, along with low-level (which REL does not).

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