Originally Posted by SAM64
If so, does that mean essentially I could be wasting 170 watts out of 200?
No, most inwall volume controls are variacs, not rheostats or potentiometers.
True, but that doesn't mean that they don't waste power output capability of the power amp they are used with.
Let's say that a power amp is putting out 10 volts, and we have both a variac and a potentiometer connected across it, and they are both set for 50%. output. The output voltage of each is 5 volts.
The potentiometer will present a far lower resistance load to the amp, so it will be wasting quite a bit of power.
Like you say, the variac will act like a variable step-down transformer, and not dissipate very much heat. It iwll have only minimal losses.
But, there's a 6 dB loss in both and you will have to run the output of the power amp at twice the voltage that is delivered to either load. In some sense the power output capability of the power amp is being lost.
The practical benefit of the variacs will be that you can run more speakers in a system based on them, because there is far less power loss through the volume control for every speaker that is driven.
Each variac output draws less current from the amp so you can have more of them before you are loading the power amp with a load that has too low of an impedance.