It's well accepted that the frequency response is a major criterion in establishing a listener's preference. That alone justifies EQ.
Can a headphone/IEM already fit your preference? Of course that's possible, but it's statistically way more likely that there is another response you'd like more. The headphone might need a complete re-tuning or maybe it only needs a subtle change in one place to get your favorite response. Anything is possible but just by mentioning 2 different IEMs you think you prefer without EQ, It seems like there is a problem. It's so very unlikely that both are your preferred signature, as they are already noticeably different.
Just with that I'm very tempted to question what "custom-tailored" actually means in this case. Don't get me wrong, if you don't know where you're going, for some FR that will be more pleasing to you, there is also a vast array of FRs that would make the IEMs subjectively worst for you. Nobody said EQ was easy.
It's like giving paint and a brush to someone, sometimes you'll love it, sometimes you would have preferred no work done at all.
Both IEMs have a fairly typical response(different but typical), and I wouldn't be surprised if you happened to enjoy your music using both as they are. But I'd be even less surprised to learn that there is still one response you like more. Aria seems easier to EQ with its smooth response so you probably should fool around more with that one for now. I cannot talk about you and your ears, but I can talk about what I would probably prefer(which is subjective and not in a any way something you must prefer. I just happen to know myself a little when it comes to FR as I've been fooling with EQ and IEMs for a LONG time):
I like bass rumble but I don't like extra energy above 100Hz, so based on online measurements and assuming I'm able to get a nice seal, I would probably attenuate a fairly large area centered around 150Hz or a little lower by... let's say -3dB for a first try.
Then I would probably fool around 4kHz in a really narrow band because it's one of those frequencies where a little too much kills me but too little kills the energy of the track. So trying to find what's just right for me is usually beneficial to my overall enjoyment.
And really that's it because the original FR seems clean. Most people would probably wish to lift part of the treble, but I usually don't take any risk with high frequencies and at the very least I do not rely on measurements for tuning high freqs.
EQing the U12 might be trickier. Some of the fluctuating parts in the upper range might not feel like much subjectively, and might not be what a graph shows them to be. Without knowing the reason for those variations on the graph I just googled, there is a possibility of things going very wrong very fast when trying to straight up compensate for it. I'd probably have to fool around with tones and EQ, but I have IEMs with somewhat similar trends that I mostly don't touch in the end because it's trouble(needs EQ but in such a very local way that a slightly different insertion ruins everything by shifting the frequency of some peaks and dips.
I still would do the same as with the Aria for the bass boost bleeding too much above 100Hz for my taste. For the rest I can't say for sure.
About the EQ as a tool, some people find dramatic differences between one EQ and another one. Personally so long as I have parametric EQ, I'm happy. I did purchase a "pro" EQ software and I love the freedom and versatility, but I really did that because the free parametric stuff I was using with foobar before would crash all the time while I was tweaking a response. So frustrating to spend time on a specific EQ and to lose all the data in a crash, several times in a row. ARRRRRghhhhhhhh!!!!
Anyway my initial point wasn't to whine but to say that some people seem care about the "quality" of the EQ used. I agree that EQ on DAPs is often crap or very insufficient, and non parametric stuff rarely make me happy. Beyond that, I don't know what people feel that I don't but maybe it's part of why you can't be satisfied?
You brought up volume change and of course that's very important. There is nothing you can do about it really. Just stick with an EQ and stop going back and forth between bypassed and ON. Give it time, forget about it, get used to it, and see if something seems annoying or lacking. Then go back to EQ just to find in what frequency range your issues is located. Once you have some idea about the area, you can try something, give it some more listening time to get used to it and so on. For me a general tuning takes a minute now that I have a fairly good idea of what I like, but the fine tuning part can stretch over days, weeks, or just never really end(maniac+lack of skill= many hours of work
).
And as posted above, you do have to mind digital clipping and secure enough headroom with the general gain. That, try to use less amplitude in your EQ than you first think you want(I'd say to cut it in half right away). And be careful with subs and high freqs as you might bring up more distortions than actual music. Overall, if your ears are happy, all is well. Don't trust any claim of what's right or neutral, you have your own head and your own concept of neutral sound based on your specific body(that advice is stupid if we're talking about EQing speakers because they do have a neutral that works for everybody without massively damaged hearing).
