When it comes to a REL Sub Bass System we want the REL to work in harmony with the speakers, reinforcing bass, not cancelling it. In other words, we want the drivers in the speakers and the REL to move forward at the same time and move backward at the same time. When this happens, it results in an increase in sub bass volume and the pressurisation in the room and they are in phase. The opposite is if the speakers and REL are working against each other causing cancellation of low bass. We call this out of phase.
The phase control on a REL has two positions, 0 and 180. One of these positions will set the REL in phase with the speakers, and one will set it out of phase. The labels 0 and 180 are just labels and tell us nothing about the rest of the system, only if the REL is working with the speakers or not. Whichever position results in the loudest and fullest sub bass, is the correct position.