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Greeni, there are no ultra linear headphones. The response of headphones is purposely tailored to give a 'pleasant' sound in the ear, due to the amount or lack of coupling of the headphone to the outer ear. Each manufacturer 'tailors' the sound to its version of how it feels the sound should be. An 'ultra linear ' headphone would sound awfull!
Perhaps you should inquire among audiologists for what earphones they use for testing. Those should be 'flat' headphones, but not very musical.