From the same link:
A volume control adjusts the output levels....all the gain control really does is control the sensitivity of the input (RCA jack or XLR) jack on the subwoofer...The maximum output capabilities of the subwoofer never change.
He is correct in the quote above, which is also what I stated. He contradicts himself when he states "A gain control adjusts the output levels
*relative* to the input levels." That just can't be true if "The maximum output capabilities of the subwoofer never change
". It adjusts the input
relative to the output, which is just the opposite.
An amplifier multiplies the voltage of the input in order to amplify it. The output always uses the same multiplication ratio, and does not change no matter what the input is.
In a receiver, the volume control adjusts the output of the pre-amp section
. The amp in the receiver, like all amps, multiplies this by a set value (let's say 10 times) determined by its circuit design. When you use a pre-out jack run to another amp, such as a subwoofer out jack on the receiver, the volume controls that as well. Now let's asume that the subwoofer's amp amplifies twice as much as the receiver's amp (20 times the input signal). This means that the subwoofer would be louder than the rest of the speakers! The gain control allows one to adjust the input
(it actually pads it down, with 0 being 100% attenuation and 10 being zero attenuation) so that the sound level matches the rest of the system, and one may then control everything with the receivers volume control, and amplification will be even across all channels. the subwoofer amp is still multiplying 20 times, and the receiver's amp 10 times. That never changes. That is why one never changes the gain setting on an amp after it has been set properly.