Joe, the first point is that the volume control on the receiver doesn't do what you think it does; it doesn't directly increase the watts which are available for the speaker to use. It's a variable resistor which varies the percentage of the voltage which comes in to the receiver(e.g. from a CD player)which gets sent on to the following amplifiers, either the built-in receiver amplifier for the speakers or the amplifier inside a powered subwoofer. The voltage from the CD player will vary widely, depending on how loud the music on the CD is at the moment, from less than .01 volt to a maximum of maybe 2 volts, but lets say that it's exactly 1 volt at this point. Next, lets say that at a certain point on the receiver volume control(not necessarily half-way)it lets 50% of that 1 volt through to both the speaker amplifier in the receiver and the amplifier inside the sub. If the music on the CD got louder and the CD player outputs 1.5 volts, at that same 50% volume control setting both the speaker and sub amplifiers would get 50% of 1.5 volts(i.e. .75 volt instead of .50 volt), and both of the sets of amplifiers would amplify that increased voltage, so both the speakers and the sub would get louder. If on the other hand, the CD output remained at 1 volt, but you turned the receiver volume control up to a point where it let 75% of the voltage through, both amplifiers would again get the increased voltage(e.g. .75 volt), amplify it, and again both the speakers and the sub would get louder.
The sub output control on your receiver, which you say is numbered 0 to 20, is a separate matter which adjusts the sub up or down in relation to the speakers, but as described above, both get the increased voltage for their amplifiers if you turn up the receiver volume control and so both get louder.
