Subwoofer Phase Control

Cos REL doesn't want people doing diligent bass control in their room and practical use case of integrating sub and mains. Cos a continuous phase control with some sanity check RTA measurements leads to a better blended speaker/sub response in your room. Taking that away seems counterintuitive but I guess REls magically integrate so there's nothing to worry about /s @discus99 

The phase control certainly impacts the integration and I've played with it quite a bit. But I can't say I've found a setting that I consistently prefer. The tonality changes don't translate into a "better" integration, just different. It's probably more important for multiple subs located away from the mains. 

If your goal is flat frequency response in the bass, then yet, a continuous phase control (or one with small increments, such as 1° in the SVS app) definitely is a help and will give more positioning options. 

Phase adjustment is frequency specific; I assume it is set at the crossover point for most subs. This allows one to marry the frequency response between speakers and subs at that point. One could do it by ear with a test tone, but then the user is just closer to having acoustic measurement equipment. I assume this complexity is beyond what REL wants to market as novice-friendly setup. REL also instructs setting both volume and crossover point as low as possible, again done by ear. I think that makes phase adjustment less meaningful. The ability to adjust polarity is more vital in my experience with both REL and JLA. Even with phase matched the sub is playing many cycles behind the speakers. 

-3dB subwoofers are an extension of the room they’re in. A subwoofers adjustments aid in their integration with the main speakers.

The easiest way to locate and mark your rooms unique multiple optimum placement locations and the first step after unboxing a subwoofer is the subwoofer crawl. 

A variable or 180 degree phase switch simply allows the user to better synchronize the subwoofer driver with the speakers drivers.

Even the most ambitious room equalization or optimization parameter adjustments can do little to ameliorate the improper room placement of a -3dB subwoofer capable of +dB attenuation below 20Hz.

A -6dB sub-bass system woofers roll off long before they can excite a rooms standing wave bass modes giving them the advantage of almost limitless placement, at the expense of the delicate extra low frequency information found entertaining on many recordings.

From their brochure, [REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)]

You cannot correctly integrate a sub without variable phase control for many a speaker.

It is all foolish noise when a dude claims his sub just dovetailed in with  a 0/180 flip switch... If you listen/audit his rig with a discerning ear,  his noise shall get exposed.

AFAIK the "phase" control is really just a delay, with a theoretical 180 degrees being the equivalent of 1/2 the travel time of the upper frequency.  So for a subwoofer with a 100 Hz max 180 degrees would be 1/200 = 0.005 milliseconds (if I did the math right).  90 degrees would be half that or 0.0025 milliseconds.  

If you are using a HT processor this should be dealt with by the speaker delay calibration.  

If using this with high passed speakers the correct setting is the one when the inverted high passed speakers + sub have the deepest null at the crossover.  Otherwise it should be set for the smoothest response below the low pass filter setting. 

In my experience, this feature really helped me integrate my REL (some of their models have phase control) in a high passed speaker setup that I have. 

what I saw (using REW) was that Phase control removed the "dip" at the integration frequency of the sub with my monitors.  funny, I accidentally discovered this as I was wanting to remove that dip and when i switched the phase switch to 180, then the dip disappeared!