MEST MKII

I love a wide soundstage, but don't think that really exists with headphones so I care much more about imaging (speakers are another story though). I'm with you. I love my HD800S, but my MEST MKIIs consistently bring me to tears.

UM MEST MK2. It took me a while to realize, but the bone conduction makes a huge difference to me. Just magical.

+1 I prefer it over my annihilator and mest mk2

I like 64audio solo, probably endgame for vocals. I prefer those over mest mk2

Mest Mk2 has the perfect balanced bass for me. Not too much boomy but feel tactile and high quality

Look for something from a company that implements bone conduction well (Unique Melody, Penon, FlipEar for example). It pushes IEM soundstages to their limits. MEST Mk2 has an absolutely massive stage. More expensive options exist that bring similar features.

Just to add you are completely on point with hardware limitations. The MEST Mk2 really can’t add much more bass or the BCD/DD will get out of sync for example. BCDs won’t ever be reflected on a graph, but have significant impact on listening experience.

As someone with HD800s and Mest MKII, I share a lot of your sentiments. What helped my HD800s a lot is listening to them in quieter/silent environments. All the little environmental noise from fridge, cars outside, AC, fans etc while not really noticeable, all kind of mask details and make the headphone sound worse. That's why IEMs feel so effortless when it comes to sound quality as they block out a lot of external noises. Btw I also have small canal openings and have to use S/SS eartips. Luckily Moondrop Spring tips are very comfy to me and doesn't cause any itch.