EQE is an iOS runtime modification, so it is not restricted to a single app. It works with all of them.

Spotify, SoundCloud, Apple Music, Hype Machine, YouTube, Snapchat, WhatsApp voice memos, you name it. EQE does not discriminate.

EQE is fully parametric, with support for up to 200 simultaneous biquad filters. Here are a few more of its features:

• Frequency response curve
• Manipulate individual channels
• Lowpass, highpass, bandpass, highshelf, lowshelf, notch, and (of course) EQ filters
• Changes to audio happens in real-time (unlike a lot of other equalizer apps)
• Lua scriptable to the core (documentation here)
• Command-line ncurses UI (so it can be controlled remotely via SSH)
• Classic graphic EQ controls, for those who prefer simplicity over control

EQE also has the ability to remember your playback history, akin to Last.FM's audioscrobbler.

This also works seamlessly with every app. Set it once, and you won't have to think about it again. Keep your playback history private to yourself, or share it online via eqe.fm.

EQE requires a jailbreak to install. Open Cydia, and search "EQE".

EQE is compatible with a wide range of iOS devices, eight of which are permanently jailbreakable on their latest available iOS version:

iPhone 4: Pangu for iOS 7.1.2
iPhone 4S: Phoenix for iOS 9.3.5
iPhone 5: h3lix for iOS 10.3.3
iPod Touch (5th generation): Phoenix for iOS 9.3.5
iPad Mini (1st generation): Phoenix for iOS 9.3.5
iPad (2nd generation): Phoenix for iOS 9.3.5
iPad (3rd generation): Phoenix for iOS 9.3.5
iPad (4th generation): h3lix for iOS 10.3.3

For the rest, you have to be on a specific iOS version. More info can be found on canijailbreak.com. Newer devices can also be purchased on /r/jailbreakswap.