Welcome here ...
I think that there is no "perfect" headphone...There is only a S.Q. /price ratio...
A balanced headphone for me is difficult to define but very difficult to bought.....
Did we spoke only about a balance between 50 hertz and some high precise hertz ceiling?
Or do we spoke about a headphone able to give bass with depth and clarity near 30 hertz and extension with the most exquiste non fatiguing high?
Or do we spoke about mids frequencies rendition so beautiful and enebriating as in the Stax Sr-5 gold for example but with a high roll off in the bass ? For me the Stax Sr-5 gold is unbalanced if i refer to a speakers system in an acoustically controlled room as reference, but for Stax designer this headphone is well balanced and for me it "appear" to be so at first when i bought it...But knowing what i know now i will never describe it as balanced because of his limiting window with a high floor limit over 50 hertz...
For me balance means an headphone with no boominess at all, deep extended bass, non fatiguing sharp highs frequencies, natural timbre and voices, natural does not means beautiful here ( mids of the Stax are more beautiful than natural) ... Then not only beautiful mids frequencies as in the Stax SR-5 gold case but beautiful in the deep bass too ... I encountered it only with one headphone and this headphone is so hard to set right i cannot even recommend it to everyone... The Akg K340...
My point here is an headphone unable to give bass near 30 hertz with clarity and extension, and mids completely separated from the bass and violin highest note with a color to die for is not balance and cannot be...Most headphones are unbalanced for me, at least my 9 past one were each one was unbalanced in his own way...
A singular driver headphone with his own frequencies limits specs may be called "balanced" by the designer and reviewers...But if this headphone cannot give non fatiguing highs, or deep bass heard by my feet and body, by bones resonance effect, as with speakers by the waves pressure felt by the body , it is an unbalanced headphone, so balanced it is supposed to be in the marketing publicity with their own fixation around the mids equilibrium ...
Yes i am very hard and demanding in headphone evaluation because i come from speakers experience in a dedicated room and i compare headphones with speakers balance in a room and i ask an headphone which will be described as a speaker-like balance soundfield ... ... It is only my opinion though and i am not at all an expert in headphones nor in speakers ...
But i am in heaven now with a truly balance headphone after 6 months of optimization around it... Then for me a truly balanced headphone is a rare gem...not a common products at all... And if an headphone sound only inside the head with an headphone feel instead of a speakers feeling outside or around the head it is unbalanced headphone for my ears...