Thanks for your post and appreciation...
And you are right about the complexity of these concepts : "balance" is complex concept because it is not only a measurable quantities relating to some frequencies ratio it more so and importantly an acoustically felt impression associated to a soundfield 3-dimensionalities dynamics.. ...
And after timbre and distortion rate , the soundfield most important characteristics is not only the traditional two factors : imaging differentiation and soundstaging global dimensions but also Holographical VOLUME dimensions associated with each sound source or instruments or playing chords ... These three factors constitute "immersiveness " quality of an headphone if the timbre is right and the distortion factor low...
Big +1 @mahgister It’s not only about balanced tonality but also imaging and soundstage. For me, if everything sounds like it’s inside my head it’s very limiting and not enjoyable, so overall “balance” can mean different things to different people, as always. But I have two pairs of Hifiman open-back ‘phones (HE400S and Arya Stealths) and find them to sound very neutral and balanced while also offering very good “out of the head” imaging/soundstage. Of course your choice of amp, DAC, cables, etc. will significantly influence whatever “balance” you ultimately achieve as well, so it’s obviously not just about the headphones. It’s the whole “the chain is only as strong as the weakest link” thing that’s pervasive and unavoidable throughout audio.