Here's are two incredibly good (imho) EQ apps for your iphone, same dev...
https://apps.apple.com/us/app/highamp-mp3-music-player/id1359487085 - or -
https://apps.apple.com/us/app/highstereo-mp3-music-player/id1164322969
10 or 30 band, + preset & programmable curves, with independent rotary Bass & Treble controls for on the fly granular adjustments.
The highlight being the rotary controls - quickly adjustable on top of an EQ curve.
(some marketing prose from the dev)...
- Better sounds quality with Bass, Treble and Equalizer
- All music collections including Youtube, Apple Music, iPod library, files in your PC and files in Clouds.
- Supports various audio formats such as mp3, flac, ape, opus, ogg etc.
- 100K radio stations.
This app's transformed my own TWS experience. I've no affiliation with the product, .and if there are better EQ apps out there for the iphone, somebody please tell me. I'd like to know about it. Fwiw.