These controls have been around for 30 plus years. The original manufacturer was Soundalier and the most popular unit was the 816122-2. This was an 8-16 ohm unit with stereo capabilities. These also came in voltage division controls which are used primarily in commercial installations with very long speaker runs. Today’s volume controls have switches on them to keep the load within the amp’s capabilities depending on how many volume controls are hooked up in a parallel manner, keeping it above 2 ohms and preferably closer to 4 ohms. Volume controls are transformer coupled; that is to say that the load which is the speaker is not directly wired to the amp. It is coupled through one of a multiple of taps on the transformer which varies the volume of the connected speakers. The beauty in this design is that the amp does not sense a change in the load and it does not affect other speakers on the same amp due to the fact that the amp only sees the primary side of the volume control and what happens on the secondary side which allows for volume controls has no effect on the amp.