I found this app way back around when iOS 4 was released and Im still using it on iOS 9.2.1 and will continue to do so on all the next versions. People are talking poorly about this app BECAUSE THEY DO NOT KNOW HOW TO USE IT. You have to set the headroom to safe because on ANY amplifier, it will distort and clip of you drive it too much. Thats a hardware limitation. Start off with a slight W shape equalizer or a squished U shape and adjust from there. Increase the bass and the treble sides up and then the middle around 500-1k Hz area to boost the vocals. This app has the potential to make any set of cheapo speakers or headphones sound WAY better if you set it right. This has been my primary music player for the past 5 years or so and its what convinced me to switch from my Sony Walkman MP3 player to iOS devices for music. Highly, highly recommend EQu.
SteelFlexInc about EQu - the quality equalizer