Continuous Phase Adjustment in Subwoofers

You're right, a continuously variable phase control does just what you said. All the "Standard size" Rythmik Audio plate amps include a variable phase control, 0 to 180 degrees (which provides zero to 16 milliseconds of delay).

REL advocates their sub integration through high-level (speaker-level) connection, which feeds the subwoofer the exact signal the main speakers receive including the same amplifier voicing and timing. This natural alignment of gain and signal characteristics helps eliminate many of the integration complexities that would normally require continuous phase tweaking. 

Hope this helps!