A continuously variable phase knob is a good thing to have because the receiver's sub distance setting may not be granular enough. Half a foot at 80Hz is about 15 degrees and that could be the difference between good and excellent.

When are you coming over here to witness how marvelous your sound panels are at converting acoustical energy to thermal energy?

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated