Electric to acoustic transducers (loudspeakers, and even more so headphones and IEMs) are confusing because of the complexity of their functioning and interaction with human ears. Much research has been done, but no one can provide definitive answers, especially on account of the variability of head/ear shape and ear canal geometry from person to person. Crinacle and Oratory1990 and Rtings have done great work recording the frequency response (FR) of transducers and making these data accessible to the public. They can help rule out IEMs that are awful, but even at the current time, you would have to experiment for yourself to find out which IEMs are well suited to your requirements and sound good to your ears too.
Tuning the room is mostly about minimizing anomalies in low-frequency reverberations, wall reflections and cancellations. Please understand that the ideal FRs of headphones and of IEMs would not be flat at the DRP (eardrum reference point) if they are neutral, unlike loudspeakers whose anechoic FR is ideally flat in order to be neutral. The reason for this is the selective (frequency-dependent) amplification of the sound waves in diffracting around the head and interacting with the outer ear. So a flat/neutral FR sound from a loudspeaker winds up being non-flat but perceived as neutral at the DRP. You can search ASR and the web for "HRTF" and "ERTF" for more info on these effects. The Harman target has a good philosophy for the initial selected shape of the curve (corresponding to good (neutral) speakers in a room). The Harman target curve is preferred by a majority of people, but that majority is probably around a two-thirds majority, and there are a lot of people who find IEMs close to the Harman IEM target unsatisfactory or not as good as some other FR. And many researcher websites may show their own preferred curve, which may differ somewhat from the Harman curve. I myself do not care for so much bass as the Harman target, and also prefer less accentuation of the clarity region, and I believe I would prefer the FR of the Final B3 but with the bass response of the Final B2, but that is just my ears and the genres of music I enjoy. The Harman IEM target is preferred by a bigger percentage than those who prefer Free Field curve or Diffuse Field curve. So in the end it is better to experiment for oneself, with the understanding that nothing that you pick or end up with is going to be perfect no matter how much money you spend on it. The only IEM on Crinacle's ranking list that I remain curious about is the Hidition Viento.
In my opinion, a good way to experiment is to pick a few lower-cost IEMs that are described as neutral and are rated well by users who share your taste in music, and whose FR has also been measured by Crinacle or Oratory1990. Pick well regarded IEMs that have somewhat different FRs, so not all of them adhering to Harman target which seems to be more popular nowadays with IEM designers. Listen to these IEMs (and ones you already own) with the music you prefer or are working with, doing volume level matching as well as you can when comparing different IEMs, and also compare with the real-life sound of your guitar and your music through decent speakers. Eventually, you will have an idea of which IEMs are more neutral and suit you better. At this stage, you can also apply software EQ to one or more IEMs in order to mimic the FR of other IEMs you do not have or to make it match various target FR curves. Next you can go back to Crinacle's list for example and look at the FR of IEMs from his higher tiers and find IEMs he describes as neutral or mildly V-shaped, and compare their FR with the FR of the budget IEMs you found suitable. You can stick with the best budget IEMs you tried, or you can now pick a higher-priced IEM (but within your sensible budget) to try with a similar FR which has a higher probability of meeting your requirements. Following this, you can improve the FR further for neutrality for your ears by using software EQ (you can experiment with the EQ). Nobody can correctly tell you which FR is exactly neutral for your ears, and given the flexibility and adaptability of brain/ear function, the lack of exactness does not matter as much as you might think. The FR is the dominant aspect determining fidelity, but low-distortion drivers and an ill-defined attribute termed detail retrieval/resolution are also important. When making mixing and mastering decisions about your music, it is also important to know what music reproduction systems the majority of your target audience will be listening through. Check ASR and the web for info on and discussions of Floyd Toole's "Circle of Confusion".
ASR is an excellent starting place to deepen your understanding of music reproduction. Here are a few links (out of many) that you may find pertinent or interesting:
Biomechanics of Human Hearing
Objectivist friendly and wallet friendly IEM?
If price were no object, which IEMs could beat the Etymotic Research ER4SR for neutrality and detail?
IEMs close to Harman curve?
Harman curves sound different? (room x headphones x IEMs )
post your headphone or IEM tier list
Looking for new IEMS
Your favorite IEMs?
Reflecting on Audiophilia - some personal stories (see the link in the OP to the three-part article on the Harman target)
