I ran into this problem when searching for a way to increase mic gain on my iPhone. The problem is "sandboxing." Some features of the phone are not available for app development. Example: The camera was not accessible in earlier versions of the IOS. Newer iPhones lack a hi-res digital to analog converter. They can stream digital audio to bluetooth or to the Lightning port. Yes, they can play audio through the built in speaker, but not in hi-res. Thus, equalizers are meant to modify a digital stream to be sent to an external DAC. Because of this you cannot use an equalizer app on analog audio. Lightning earbuds have a DAC built into the plug housing. You can easily get a Lightning to analog adapter which has a 3.5mm analog output. This could be used to connect an external equalizer. Otherwise, I don't think there is a solution.