EarStudio 4+

Buy this if you truly care about audio fidelity! Chances are your earbuds, headphones or speakers aren’t delivering the tonal balance that you prefer and this app includes a 10 band EQ and a bunch of other settings that make you able to get the best out of your gear. Don’t believe those telling you that an EQ is bad for good audio. Check the Fletcher-Monson curve and why using EQ makes a heck of a sense. Check how your hearing deteriorates as you ages and tweaking the treble can rejuvenate your listening experience. Or just check how the majority of audio gear is with flawed frequency response. And besides, you might have your personal preference, perhaps you need a bit extra deep bass for your EDM listening...and so on. Professionals use EQ all the time and consumers are told by some companies its bad to use...no, use this app and tweak the sound and you will realize what you have been missing.

Having such an excellent EQ in the audio chain is essential for getting the most enjoyment out of the music IMO. The iPhone should have this built into ios, instead of the same EQ settings that came with the original iPod.
Suggestion for developers: Custom Headphone profiles which could change various parameters all at once.

It took me forever to figure out I’m how to save a custom EQ profile. I’d set the frequency levels I wanted, go back to the profile menu, then my custom profile would reset to FLAT. I finally found out in order to save the profile settings, you have to go back to the profile menu, then long press on the profile you want to save, which is the same thing you’d do if you want to change the profile name. Then tap done. This is a confusing function. Instead of having to go to the profile name editing pop up to save, it should save as soon as you exit the frequency adjustment menu.