Thank you for your answers.
@Matt, I am not looking for a calibration of the absolute (acoustic) playback level, but (like I have for example in foobar2000) having the option to make MC's internal volume take 1-dB (or 0.5-dB) steps.
From a perceptual point of 'view' (that should probably be 'audition'
), taking dB steps is more intuitive, since they mean a constant subjective loudness step independent of the absolute level from the loudspeakers. MC's current % steps mean hardly a subjective change when you're playing close to 100% (a 1% step from 100% down to 99% is less than 0.1 dB down), but mean a larger change when you're playing around 10% (going from 10% down to 9% means reducing playback level by about 1 dB).
Also, I do not think that MC's % scale needs to go. I would be perfectly happy with the option of having the % steps (i.e. the change of the Volume % upon issuing the Volume Up / Down commands) depend on the current % value. For example, when playing at 100%, this would make 'Volume Down' result in 89%.