I have a few audio player apps on my iPhone, but the one that seems to be my regular go-to is Neutron Music Player. If this sounds familiar, it may be because I previously provided my thoughts about this same app for the Android platform back in August 2022.
What amazes me is how similar the iOS version of Neutron operates compared to the Android app. Given that Apple has traditionally made it quite challenging for developers to make players work across the two platforms, I find that feature-for-feature Neutron remains fully functional for even the most advanced "audiophile" user.
Fair warning! Neutron is not for the lightweight casual music listener. The sheer number of options and features will be overwhelming to get started. And the UI, while completely functional, is certainly not par for the course compared to other players that seem to put more emphasis on style over function. But for those with a pair of golden ears, you will appreciate the attention to the smallest sonic details that Neutron can pull from your music files, and the incredible number of DSP options, while the no-bones interface is still simple enough to navigate.
Support is there for just about any flavour of audio types, including FLAC (requires local file transfer to the Neutron folder on your iOS device, since iTunes does not work with FLAC). Graphic and parametric EQ modes are available, both fully adjustable. A decent compliment of additional DSPs let you tailor your iOS device to your ear's content. And with an external DAC connected to the Lightning port (you will also need the Lightning to USB Camera Adapter - darn you and your $$$ accessories Apple!), you can gain the full potential of your connected speaker or headphone system, without being limited to the 16bit/48kHz DAC that Apple has provided. Bit-perfect Hi-Res support up to 32bit/768kHz can potentially be realized with capable components. Stereo bliss!
There are some improvements that could be made, ideally having both Artist and AlbumArtist browsing would be a good start (currently you have to choose one or the other)[2023-07-25 Edit: The app developer has commented that this is not possible in the current design, and is. It likely to get changed]. Adding support for the "sort" tags would really be icing on the cake for me for the library [2023-07-25 Edit: This is also not on the development roadmap according to the dev]. And maybe a UI update someday would be nice, but ultimately not a deal breaker.
Find the Neutron Music Player download link to the App Store here. Note that Neutron is a paid app.
2023-07-25 Update: Exchanged direct email with the Neutron Player developer, and it appears that two of the key library management updates that would make it more functional are not planned.