Does a REL subwoofer make the speakers' job easier?

@hilde45 Thanks for the note! It seems there is a lack of common knowledge on how phase adjustment is performed with a variable phase adjustment.  Maybe I'll write one up on that too.

If I were the type of person to give for-profit businesses the benefit of the doubt regarding what seems to be neglectful, I might think there could be something related to the high level connections that Rel likes to include. I'm not an electrical engineer with expertise in audio electronics design, but maybe there's something innate there with matching phase? In other words, if you use the option for your sub to tap into your power amp outputs or your speaker wire signal, does that automatically align the phase of your sub to within 180 degrees of your speaker woofers, thereby providing some justification for the two position phase switch they use? I highly doubt that is the case, but it's something that has crossed my mind as to why Rel and others others have lacked variable phase adjustment controls for so many years, and still do, even on some really expensive products. Even if that were so, there's a lot of people that want to use the line level inputs, and they should be able to have variable phase adjustment like Rythmik provides... And these companies themselves should provide education on how exactly to perform the adjustment process.

@gladmo: I time after after time see and hear people laud REL for their high-level connection design. Well, Brian Ding provides both high-level AND line-level connections in the non-XLR/Standard Size versions of the plate amp of his Rythmik Audio subs (the face plates of which are clearly pictured on the company’s website)..

But he also includes a continuously-variable phase control, which he labels "DELAY". It provides from 0 milliseconds of delay (0 degrees of phase rotation) to 16 milliseconds (180 degrees of rotation), and anywhere in between. The phase switch included on other subs (providing either 0 degrees of rotation or 180 degrees)---including on all RELs’---is a complete joke, of very little practical value. And is in fact an insult to one’s intelligence.

The notion that a sub doesn't require a continuously-variable phase control because a sub is either in phase or is not is a bizarre one. If your cross-over frequency is centered at, say, 100Hz, both sub and main speaker are reproducing that frequency (the high-pass and low-pass filters creating a slope---a decline in output---of the drivers involved). If the wave from both reaches the listening position at the exact same time, their combined output creates a flat frequency response (when well designed and implemented ;-). If a 100Hz tones reaches the lp with the waves from the sub and main speaker completely out-of-phase (180 degrees of phase rotation, or delayed in time 16 milliseconds, commonly referred to as opposite polarity), the two waves will combine to create a deep null in the response. A smaller degree phase mis-alignment will create a shallower null. This is not opinion, it is a fact, one of course well known for a century by all loudspeaker designers. Speaker designers have to provide the same phase alignment between the bass woofers and midrange drivers in their full-range loudspeakers, so as to create a flat frequency response. The alignment between a sub and a loudspeaker is no different. Why WOULD it be?!

I was amending my above post, and ran out of time before fully completing my thoughts.

As we know, a continuously-variable phase control is very uncommon on subwoofers. All the control on the Rythmik subs does is provide either no delay, or up to 16ms of delay. 16ms of delay does exactly the same thing as moving the sub 16 feet back in space (approximately; sound travels at about one foot per millisecond).

Audiophiles have long had to move their sub(s) around, searching for a location in the room where the sub and main speaker seem to best "blend", where the sub doesn’t sound separate from the loudspeaker. That effort involves two completely different issues: 1- the phase relationship between sub and speaker; and 2- the interaction of sub and room. Unfortunately, the two are often in conflict with one another; the best location for one issue is the worst for the other. That’s one reason integrating subs with speakers has always been such a hit-and-miss proposition.

The worst place to locate a sub (or speaker) is where the room is creating a "mode": either a resonant mode ("’room boom") or a null ("suck out"). What a continuously-variable phase control allows one to do is find the best location in a room in regard to the sub being in a non-mode location, and to then use the phase control to align the sub in time with the loudspeaker, rather than moving it physically (to a location where a room mode exists).

Room modes are completely a function of room dimensions, and you can see where the modes are most likely located in your room by putting its dimensions into one of the room mode calculators easily found on the ’net. Place your sub(s) where room modes are at their lowest level, then use the phase control to align the sub and speaker. An elegant solution to an old problem! ;-)

Unfortunately, it’s not that simple; the long wavelengths of low frequencies (often longer than the rooms dimensions)---upon reaching a room boundary---reflect back into the room, meeting other waves and combining with them to create an increase in response (when in phase) or a null (when out of phase) at the location where the waves meet. It can therefore be a complicated mess, one difficult to completely resolve. A continuously-variable phase control is but one tool to use in addressing the issue of sub/speaker/room integration. It's therefore no surprise that many critical audiophiles have never been fully satisfied with the results of adding subs to their loudspeakers. For them, Richard Vandersteen has done the hard work for you. Just buy his Model Seven or Kento Carbon. ;-)

@johnson0134 

Just a follow up! 
Did you eventually add the REL dual subs to your system. If yes,  then how  was your first-hand experience of adding the REL subwoofers to your system and how it changed the sound for you and any recommendations! What models of REL did you add?
Thanks.