It's generally ignored because the ear/headphone makes a HUGE difference, whereas the DAP makes a tiny difference in the grand scheme of things. I'm not buying a player to resolve the deficiencies of a cheaper IEM; I'd rather get the more expensive IEM first if audio quality is my goal, then when I just want to eke out what little left there is, get the DAP........
This is a subject with many choices, depending on what you want. So the first question is what is your goal?
One perspective is that it is a System, and everything matters. If you go down the upgrade path, you need to look at what you want for the system. People can be happy with doing part, or none of the system. That is a personal decision, as different people will be happy with different choices.
True that the earphone makes a big difference. But also true that if your source is deficient, you will handicap the system.
- You can choose to improve the earphone first and stop there (if you can)..
- Or if the goal is the "best" system, then you will need to do more. You need to decide what kind of system you want, as the choices will depend on this. i.e. A no holds barred system? Or a moderate system, or a basic system? Or you do not care about the system, but only want to spend "x" amount of money and get the biggest return? There are different choices for each one. There are no rights and wrongs, just what you want for yourself.
Though I think of an analogy like putting very good tires on a car. Definitely feel the difference. But if the car had a better motor, and a better suspension, and a better chassis, there would be an overall system improvement, and you need to worry about synergy.
. If you want a fully optimized car, you would need to worry about the whole car synergy, and not just the tires. Just throwing components together without considering whether they work well together, will not get you the best result.
Or deciding that an architect is not necessary to build a house. Just pick all the features you want and throw them together. I've seen the result of this, and I thought the result was very imbalanced, and not a good "home".
So it comes back to deciding what your goal is, and then working towards that. If you have not thought about, and decided this part, it is very hard to decide what to do. Anybody else's opinion is based on their goal, and would be relevant to your situation if you are clear on your own goals, and how they matched up with somebody else's.