sokogear, I'd like to make a slight correction to your statement. Everything that is measurable is not audible but everything that is audible is measurable. Sound quality is subjective to some degree and subjectivity is not measurable.
As an example, I can set up a measurement microphone, play some music and display a sonic spectrum on the computer. I can change the frequency response an see the change occurring on the computer in real time. I can go the other way and play a slow sine sweep and look at the frequency response curve. If I see something I do not like I can make an adjustment and run another sweep to see if I fixed the problem. Since I can run individual sweeps for each channel I can adjust them independently until the two channels are within 1 dB of each other from 100 Hz to 10 kHz.
As an example, I can set up a measurement microphone, play some music and display a sonic spectrum on the computer. I can change the frequency response an see the change occurring on the computer in real time. I can go the other way and play a slow sine sweep and look at the frequency response curve. If I see something I do not like I can make an adjustment and run another sweep to see if I fixed the problem. Since I can run individual sweeps for each channel I can adjust them independently until the two channels are within 1 dB of each other from 100 Hz to 10 kHz.