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.
Yeah that was a very interesting thing to learn. When my room was being tuned (after being built to a specification for acoustics) the acoustician then actually tuned in several switchable curves because it was so flat in response he wanted to make it sound more natural to work in.
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)