As it happens, there is a field of psychology called psychometrics that deals specifically with measuring subjective experience. It turns out that brain waves are rather crude for discerning small changes in one's experience. Better are rating scales targeting specific dimensions of perceptual experience. There are four dimensions: a) Valence (good vs bad), b) Potency (strong vs delicate), c) Arousal (relaxing vs stimulating), and d) Novelty (familiar vs. unfamiliar). All of our perceptions are really just a composite of these four factors much like all perceived colors are just a composite of red, green and blue (with a brightness factor thrown in).
I have used psychometrics professionally to measure all manner of perceptual experience including sound quality. Here it is important to note that there is a difference between the physical qualities of sound and the emotional qualities of sound. I always aim to correlate the two to reveal which physical qualities are most predictive of desired perceptions. Not all physical qualities are equally relevant, perceptually. Knowing the differences is what is important.
