You can certainly measure it, but the catch is that there is not always a single "correct" value. So just because you can measure what the speakers are outputting and then adjust it, it doesn't mean there is one correct output value.

A good example of this is a target curve, often used in room calibration. Dirac has a good explanation: https://www.dirac.com/resources/target-curve

(highly recommend Dirac room correction, by the way)