Typically you aim for as much of the audible spectrum as possible with as flat of a response as possible... From there you adjust for personal listening prefere

Typically you aim for as much of the audible spectrum as possible with as flat of a response as possible... From there you adjust for personal listening preferences. Some cans tend to accentuate bass a bit, others mids or trebs. Flat headphones are preferred for mixing/mastering of course. That's about as objective a measure as you can find.