[gradoexperience]

In IoS update 8.2 Apple did a stability update for audio. The sound indeed changed (headphone Grado RS1i). Bas sounds more easy (less pressure and bluriness), vocals are more prominent. Overal more detailed and in balance. Others say it sounds softer, yet I think that's because their is more control in the sound.

A few days later I thought: Could it be that Apple changed the frequency boundaries from 20 tot 20000. Because I feel less pressure in the bas and vocal are more promiment (more prominent midrange). Could it be? Or does it sound more natural now, as the music is meant? What is your opinion and explanation?

 

[zgeneral]

It could be anything? It could be your own bias. While people here use phones, I'd highly suggest you visit a dedicated apple or mobile phone forum. You'll likely get a lot more information.

Also, Apple does have release notes for different versions of IOS. You should track those down and see if there's anything related to sound in there.