The sub's volume control "trims" the output of the receiver so the sub will play at the same level as the rest of the speakers. It is "set and forget", just a control that makes one sub compatible with many different receivers.
What you're shooting for is to be able to play pink noise through the left channel, and have it measure 75 dB on an SPL meter, and then play the same noise through the sub and have it also read 75 dB. Then if you turn the main volume up or down the sub will track the rest of the speakers at the same level.
Using the auto-calibration built into the receiver will do this for you.
Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs