The answer is simple: both.
There are details and parameters you can measure but not hear easily and there are details that are very difficult to measure but that can be heard.
A loudspeaker should not sound beautiful but simply reproduce accurately beautiful sounds by transforming and electric signal into an acoustic signal.
If you accept this and accept that the artist, instrument sound engineer, natural sound, or whatever you are listening to is what you want to hear, then you can measure many parameters on you loudspeaker such as frequency, phase, distortion, directivity, energy, impedance, etc.
All these parameters can be optimised objectively but then it is necessary to listen to the loudspeakers with different specialists and in different environments.
For each new development and improvement we have a panel of specialists who listen to our loudspeakers and work with them for a period of time in their environment. We like to include some classical musicians and producers, as they know exactly what their instrument should sound like. More subjective parameters are evaluated such as: transparency, dynamic behaviour, sound stage, colouring, comfort of listening as well as personal impressions.
Then process is lengthy but necessary to validate any development or change.
Once validated, we establish a reference that needs to be matched in production for every single loudspeaker, hence the individual calibration.
People ask us if we match our speakers in pairs. No, we match components in each loudspeaker so each loudspeaker matches the reference that is as close as possible to reality and subjectively validated by hundreds of ‘listenings’.