The Level control in a REL is much like the volume control in a two channel system or processor. It increases the overall volume for all frequencies from the input. The Level control is intended to be adjusted so that REL volume is even with the speaker(s).
The Crossover in a REL is a tuneable electronic circuit that filters out frequencies above where the Sub Bass System works. These higher frequencies are not useful to provide the foundation of very low frequencies that the REL is working at. This circuit is tuneable so the adjusting the crossover will let the REL seamlessly integrate into the system and room. When increasing the adjustment of the Crossover, the REL will sound like it is getting louder, this is because it is allowing more mid bass frequencies to get amplified.
REL controls have detents you can feel when rotating them. We refer to these as clicks and use them to guide positions of these controls.
In our experience, it is best to start tuning with Level and Crossover controls at 15 clicks from the minimum setting. For a majority of systems, the final set point for the crossover will be within + or – 3 clicks of this starting point.