Turning the sub's volume control up all the way, is worse than turning the receiver's volume up all the way. I can play my receiver at -0 dB without too much trouble (I'll more than likely get trouble from the police).
But the subwoofer's volume control is designed to make it so full volume on the receiver equals full output from the sub's amp. Voltage levels for the sub-out vary from receiver to receiver. All sub maker can do is design a control to try to cover the all the variances. The right setting is the one that makes the sub produce an output level exactly equal to the main speakers regardless of the volume control on the receiver.
Turning a sub's volume control nearly all the way to get more bass is a mistake.
Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs