What Does Ref. Level Mean on Our Subwoofers?

Ref. Level or Reference Level is nothing more than a fixed volume position that can be turned on or off.
This simplifies the setup process with AV receivers, AV processors, and amps that have proper active crossovers and volume control built in. One less setting to worry about.

When turned off you can use the Master Volume setting and adjust the subwoofer's volume from -20dB to +20dB in 0.5dB increments. This can also be useful with AV gear as sometimes during the calibration process, the software might prompt you to turn down the subwoofer volume before the calibration can start. (Audyssey for example) 

With stereo gear and no real volume adjustments for the subwoofer, you definitely want to have Ref. Level turned off.