As an Apple phone and iPad user I was in this very situation. No easy way to apply PEQ in this delivery chain. Very frustrating. Then Amir did the Qudelix 5k review and all of a sudden it presented a reasonably priced Eq solution. Just mentioning this in case you missed the review.Not everyone is using them in a situation where EQ is practical. If you aren't dealing with a digital source, "just using EQ" isn't quite so trivial.
https://www.audiosciencereview.com/...qudelix-5k-bluetooth-dac-headphone-amp.17386/