Finally found an app that doesn’t have ads every 2 seconds or makes you pay for premium just to do anything in the app. This works great for boosting the volume or bass without distorting the audio. One downside is that every time you change a song, the audio cuts out for a second but that’s a small price to pay for an app that works as intended without any money coming out of your wallet.

If you are looking to boost/adjust your iPhone's sound volume there is no other way than getting a speaker or using the iOS's built-in EQ tucked away in the Apple Music settings found in the Settings app at the Apple Services section before the TV Provider. From there you can scroll down to Audio settings and it will notably adjust sound as you would expect. The EQ does not change the sound drastically depending on the iPhone model but it is the only one you can use on iPhone that works best. There are no third party apps that can change sound quality in any way. But if you want the loudest sound possible on an iPhone it is recommended that you set the iOS built-in equalizer in Apple Music's audio settings and set it to 'Late Night' where it is notably louder that any other setting. If you read this by now you should know not to trust any falsely advertising apps on the iOS only App Store as the developers can tell Apple one thing to then give the users another.

I was looking for a simple graphic EQ app that would allow me to tweak the EQ settings to accommodate the tonal characteristics of several different sets of headphones that I have (as different models of HP’s and earbuds have their own inherent sonic characteristics, that can vary from just slightly to significant) while listening to MP3 and WAV files that I have stored on my iPhone (12), and I found that this app works well in allowing me to dial in custom EQ curves for each pair of phones/buds that I have. The app was free (or at least the version I installed is the free version), and so far, after listening to music for over half an hour, my listening hasn’t been interrupted even once by any pesky adds or pop-ups, and that’s a pretty rare thing when using free apps. I rated this app only 4 stars, not because I’m unhappy with the app’s overall function or sound quality, but only because there’s no way to save different EQ settings for each of the different sets of phones/earbuds I have… but, I also admit that I’m being pretty picky in my expectations, too, considering that the app is totally free. ;)