I agree that where to start is the difficult part. Custom IEMs are not your typical mid-tier universals as they have so much more ability and one part of the spectrum won't interfere with another. And then you have to decide what is most important to you, IMO both in sound signature and in technical ability. For example, the bass of the 5-way changes quite a bit with each track, so some tracks sound bass light while others pound you mercilessly compared with something that is bass enhanced all the time. And there are bright CIEMs that present the details right to you while others present in what I term as a more natural presentation, presenting all the details within the presentation, so if you look for them you will find them, but they don't jump out at you.
If you are looking for a V shape, the LS8 and JH16 are two options I have heard, and from the demo, the Miracle may also fit the bill. The Rooth LSX5 offers a well integrated dynamic bass driver and a natural treble. They all will pull the music apart and allow you to hear all the detail. You may want to read reviews and then ask specific questions about how things compare and see if they sound like something you truly want. Not that I am recommending the aud-5X for you, but I find
this review interesting from someone that only had mid to high level universal IEMs before the aud-5X for the perspective.
See below for amps/sources.
With high end audio, there are two things to consider when pairing many thing. First is capability. Are there differences between amps? Yes, for sure there are as the Portaphile 627 is by far the best sounding portable amp I have heard (if you can really call it portable). But it is not about the ability of a single component, but more about holding other components back. For example, if I use a top tier CIEM with my Clip+ or iPhone, I am not getting the best performance I can out of the CIEM. If I use an amp with my iPhone, the iPhone limits the reproduction regardless of how good the amp and CIEM are. Same if you use a great DAC and mediocre amp. This is more apparent as you go up the CIEM chain, although some CIEMs are affected more than others, especially by the amping (most likely due to the crossovers). Second is synergy as there are some combinations for whatever reason sound better. Some reasons make sense to me, for example complimentary sound signatures while others just are.
I can agree that using a high end CIEM vs. a high end universal with a Clip+ will give a larger differential than going from the Clip+ to a DX100, but the improvement can be quite pronounced to my ears, adding quite a bit or realism from the added spatial queues and detail. The choice is up to you, but I do recommend players like the DX100 and 801, at least from a sound quality perspective. I have a source matching section in my reviews, but I now have the DX100 and RoCoo BA, so if you want more info with a particular CIEM, please let me know.
Thanks, let me know if you have any questions.