- Invest your money into headphones/IEMs, not DACs and amps. Most sources are good enough. A cheap-ish DAC/amp combination that has enough power is all you need if your phone cannot power the headphones.

- Frequency response is the primary factor in a good headphone. The Harman curves are a good initial target - you are unlikely to completely hate things tuned like that. Stay away from "esoteric" tunings that "highlight unexpected details in the music". You do not want to hear unexpected details, you want to hear the entire piece. (crinacle's frequency response measurements are a great source)

- EQ is a great way to deal with issues with the frequency response. Cannot fix all problems, but a lot. The AutoEQ project (on GitHub) and oratory1990's manual EQ profiles are your friend. On Android try out wavelet, on Windows use EqualizerAPO, possibly with PeacGUI.

- Comfort is vital. The angels singing into your ears is worth little if you have to pause every 15 minutes because something hurts.

- Good sound does not need to cost much, especially with IEMs. You can get enjoyable IEMS for ~20 UDS.

- Diminishing returns with more expensive headphones are real. Whether they are worth it depends on you - what music you listen to, how you listen, and whether you can easily afford it. Do not ruin yourself financially for marginal gains. (I have a couple of 1k-USD headphones but could live with ones that are a lot cheaper if I had to without thinking that I am missing anything absolutely essential in the music.)

- Cables are a scam. They do not make any audible difference.