GZoST

The Hexa is indeed really well tuned. I do not find it necessary to use EQ with it.

I use more than a dozen headphones and IEMs regularly, another dozen or so occasionally, and the rest are nice to look at (except for the ones in a big box in the back of my closet - those I could probably do without).

Sennheiser Amperior , blue version. Worn backwards - the cable should be on the right side.

amazon music has an interface that was designed by people who hate its customers. It appears to be tested by monkeys. Spotfy has its issues, but at least some thought went into the interface. Still tested by monkeys, though.

The frequency response is surprsingly sane.

The iPhone has a good AAC implementation, and aptX (non-HD) is comparatively worse. So why not get a Qudelix 5K which gives you parametric EQ, including the possibility to load AutoEQ presets?

HD 540 are good, and I use them to listen to classical music every now and then. There's a sweetness to the sound that is really nice.

My Stax Lambda Nova Basic also see occasional use. Apart from the lack of bass they are really good when using the oratory1990 EQ settings. Also a steal since I got them + the amp for 200 USD.

I don't get the love for the DT-48. That frequency response does comical things to most music.

Google for reviews of the HE-60. They are around, and very positive. Grado has always been Grado, i.e. I do not understand why people buy them.

Tuning headphones is hard, and changes from the "natural" frequency response of a driver have trade-offs in other areas (e.g. distortion). So headphones without their own DSP+amp are a compromise.

Having said that, there are headphones and, even more so, IEMs without significant drawbacks. I recently got a Philips Fidelio X2HR and that is comfortable, easy enough to drive, has a frequency response that allows use without any EQ, and has good technical chops for the price. With IEMs my current "goldilocks" pair are the Moondrop Jiu. Sound good enough even for the occasional listening to a symphony and are dirt cheap.

• Comfort is a core requirement, and there is no need to suffer for good sound (i.e. headphones/IEMs that are both comfortable and sound good exist)
• 20 USD already gets you a good-sounding IEM (e.g. 7hz Salnotes Zero, Moondrop Chu 2/Jiu, Truthear Hola).
• Diminishing returns kick in early and not everybody needs the best possible sound reproduction (e.g. depending on use case, music listened to)
• More expensive is not always better, but better is almost always more expensive
• Most reviewers out there are shills and should be disregarded. If a reviewer likes everything their opinions are worthless. The more flowery and vague the language of a review the more skeptical you should be.
• Frequency response is the number one determinant for perceived sound quality. Frequency response measurements alone can often tell you that a headphone is bad, but they can't necessarily tell you whether a headphone is good.
• EQ is always worth trying (but can't fix a truly shitty headphone)
• DACs and amps are a solved problem. Get something that measures well (no need to go for the best possible measurements), has the features you want and, for the amp, the power you need. The real gains are with better headphones (and custom EQ)
• The HD600 is a classic for a reason.