In order to understand the difference, you need to know how a loudspeaker is tuned. A hifi loudspeaker has bare drivers on a flat baffle. There is nothing affecting the sound between the diaphragm and a microphone. A loudspeaker designer than has a full arsenal of tools to affect it. It's like working in a chemical lab, where everything is in clear glass with no contamination and all the tools to precisely dose chemicals. If you want to alter a driver, you make a precise virtual model in software and the software spits out it's response. You know for sure that this response will be very close to the real thing. You can for example put the tweeter in a waveguide and an app will help you iterate the design 20 times before you can get all the lines parallel. You don't need to make the thing 20 times. You're done with the baffle and the thing is still not flat? That's OK, you can compensate any problems in the electric domain. You open another program that lets you make a filter out of electronic parts. It simulates a notch filter here and a shelf there and with 20 parts you have finally linearized the response. The whole thing weighs a ton and barely fits in the enclosure, but hey it sounds great. Oh did I mention that AI does that last part for you? Yup, there is an app that does just that. You give it a measurement and it spits out a circuit diagram.

Now headphones... Remember when I said that you can design a waveguide? So with headphones you are already forced to work with one. The ear cup is a waveguide, a terrible one and there's not much you can do about it. Headphone's ear cup is like room that cannot be detached from the loudspeaker. You can't take it to an anechoic chamber. You need to work with the mess that it creates. Sound is like light except everything is a mirror. A lightbulb in a torch illuminates the surroundings differently than a lightbulb in a chandelier. A headphone is a lightbulb in a crumpled tin can. The result is a mess.

Headphones are tuned by covering holes on the driver with lossy materials. You poke a hole and see what happens. Each time you change it, something drastic happens, but it's difficult to understand what. I have personally no idea what tools or software do the big guys like Sennheiser have to aid them with it. I guess a headphone designer would shed some light on it, not me. But even if you get the driver tuning right, you change the earpads and it sounds differently. More than that. You put it on someone with curly hair and glasses and that makes a difference as well :D That's because the enclosure is lossy. Very lossy. I personally don't understand why we can't add passive filters to headphones. It used to be problem of source output impedance, but now that everything is measured and reviewed, we can predict how a filter would sound. So it would be cool to see filter PCBs for headphones. It won't fix all issues because the most appalling resonances in headphones are destructive, meaning that they cannot be fixed without physical change. It can be used to attack broad valleys and bumps though.

The reason why peeps talk about frequency response and frequency response only is threefold:

First, headphones are tragically bad at it. Tuning a headphone is a nightmare as opposed to tuning a loudspeaker. They are often off by something like 10dB and it's not considered weird, that's how difficult it is to get that line flat. By comparison a reviewer's standard for a recommended loudspeaker is +/-1.5dB.

Secondly it's the only feature of sound that you, the user can affect. You can't change compression or ringing without comprehensive training in engineering. However you can push the sliders to make an EQ curve.

Lastly it has been shown that frequency response is very important. It's the most fundamental measurement that we have and our hearing agrees with the results of this measurement to the highest degree. Not 100% but still more than with anything else.

As a bonus I'll add that it's the easiest thing to measure. It doesn't require any special equipment or knowledge. The simple graph is also easy to understand. If I posted a waterfall graph instead, a common person reading it would be lost and had no idea what to make out of it.

What you call technicalities in engineering we call nonlinearities. Frequency response is also called a linear distortion. It's the kind of distortion that doesn't change with drive level (how much power you're putting in). Nonlinearities in are very important in any audio transducer. You want them as low as possible. That's because music is a continually changing drive level. Things get quieter and things get louder all the time. That's where the fun is The most important nonlinearities are the ones that happen to loudness of a particular tone, also known as amplitude modulation. Your hearing system is very sensitive to that. Amplitude modulation suffer the most from compression which is an inherent quality of any audio transducer. Basically as the diaphragm wobbles it has increasing tensions in it's suspension as well as decreasing motor strength, the further away the diaphragm is from it's resting position. This means that the stronger the wobbles, the less accurate it becomes and also the less sensitive it is. This is very bad for sound quality and it's a universal mark of driver's limitation to properly reproduce sound.

Compression can get suddenly tragically worse with phase issues. FamouS examples are Sennheiser hd820 and buchardt s400. Both have severe compression at resonances that cause phase cancellation and consequently notches in frequency response.

Last thing is the time delay spectrum. It's frequency response taken as a 'lump'. Basically one very smart guy discovered that our auditory system catches short sounds like transients as lumps of sound. You can't hear how long they last. Instead, your hearing tells you that a longer sound is higher in intensity than a short one. This lead to development of time delay spectrometry, today known for example as waterfall plots. They are particularly useful in medium and high frequencies, because they allow us to find ringing that doesn't show up in frequency measurements. It is a crucial driver behaviour and measurement for sound quality and spatial effects.

+1

I won't go back all the way to 2009 when I bought my first pair of IEMS, but the most recent problems that I remember was Surise earphone enclosure opening because there was simply not enough glue. This is dangerous because you can rip the circuit board from the driver without it.Then TRN IEM had it's connector pushed inside the enclosure. Again, nothing that can't be fixed with glue. The rest were Sure IEM cables that people want fixed because they can't find replacement. Any cable with a microphone will have these issues, because these cables are so delicate and third party replacements often don't have the functions that people want like three button control or quality of the mic.
The hygene part I don't think I need to explain apart from things that use tuning filters at the tip, which of course will get dirty, you'll try to clean it and you'll lose one in the process.

They're doing OK, although with IEMs in general it's worth it to at least be able to repair your cable. It is so thin... The prevalence of connectors in the earpieces is a very good thing although many budget IEMs don't have these parts secured properly. With IEMs it's particularly important to take care of hygene and clean them regularly and store them in the pouch. If you can manage the above, you should be able to own a pair for a few years. At least to me personally a driver failure never happened. Maybe they just haven't managed to figure out how to screw it up.

320 CBR is definitely not bad. Lossy encoding is generally quite smart. The bad reputation of MP3s comes from music websites that were encoding 128-190kbps. These were bandwidth-limited. I used to have plenty of home brew OGG and MP3 VBR CD rips that sounded just fine.
There are many expensive and unremarkable headphones anyway and among the good ones, there isn't a very big difference in transparency, even if the price can vary wildly. $150 gets you a trusty hifi pair to live with.

Meelectronics CX21, Sony MDR-EX500 in that order were my fav DD. The former is a comfy, practical, closed IEM with impressive sound, the latter is an open earphone with very neutral sound sig. Slightly too much highs, but clean and pleasant.

I didn't claim "macrodynamics", but dynamic range and of course I need to add increase of compression, which is inevitable if you force any transducer into more excursion. Since the driver is operating outside of it's optimal range, you may notice loss in accuracy. This has always been a problem with headphones. They use single drivers, which means that if you mess with the bass, you decrease fidelity in the upper range. There's no way around it. You will loose macrodynamics, as you called it, because every driver compresses the more, the further away it's pushed outside of it's mechanical center.

So to sum up, increase in nonlinearities from using EQ is inevitable, especially with boosting bass. This is especially propnounced in single driver headphones, because it affects broader range, including the region of highest hearing sensitivity. So using EQ is always a matter of tradeoffs and weighing desired and undesired audible outcome.I personally listen at low volume so I like EQ, any compression from it is not noticeable in regular use, but I can see how people can have issues with their equipment.

Yes, EQ can affect the dynamic range. Quite typical, especially for weaklings like single BA IEMS. It's generally adviced to be delicate with it. If you add 10dB of bass boost, then don't expect to come out of it without significant tradeoff. There is a reason headphones and speakers are tuned to roll off down low.

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I think you need a perspective.
Headphone FRs vary *wildly*. The question that I'd ask you is how much influence do you think the engineer has over driver tuning? I can tell you, not much. Harman Target headphones are rare because it's not an easy target to reach. Earphones and headphones are quite sensitive to tiny changes in acoustic impedance. With loudspeakers it's easier because you have all the space and mass in the world to do it. You can use multiple drivers and strap even the wildest crossover to them, with a notch filter on every peak. Now go see how you do it with a tiny headphone driver. Not to mention taking measurements of speakers is a breeze compared to headphones.
Unsurprisingly many classic headphones that are sometimes still in production predate the Harman Target and so were intended to hit diffuse field tuning (like a loudspeaker).

Meanwhile the differences between human anatomies are generally subtle.

Do they still sell them? I thought they were replaced by 800s

DIY. Find some obsolete or broken set of headphones. Repair or modify them. Listen to results.
Most headphones are very simple and easy to fix. Once I learned how to doit, people started giving me their old headsets that needed some TLC. I rotated many headphones that way. Some very high quality. Even If I couldn't keep them, I got to listen to them for a day and it was enough for me to find my own preferences and learn how good a headphone can sound. Most importantly though, I met audiophiles, who were happy to chat about their gear. Buying used gear is also very good way of meeting them.
Then there are audiophile meets. Look for them on forums. Maybe there is one in your town.