Subwoofer Phase Control

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!