Very good question. I dont think I personally have a 'ground truth' reference. Personally, It would be from prior experience (as an amateur musician and live gig/orchestra patron) of how instruments 'should' or tend to sound. Obviously not the best method. Im sure there are other more empirical measures one could use. Many people cite simple headphone measure as neutrality, low distortion, etc...Anyone with other ideas?
The one thing I know is that a lot of headphones cannot adequately replicate the dynamics of live music. This is what the Abyss does extremely well, IMO.