I meant to reply to you with that earlier post, but it didn't quote your post oh well... I agree that the brightness and harshness on earbuds or any headphones / earphones can come from the uppermids 2-5k region as well not just the treble. (unless its XinYue's Transparent MMCX MX500 that bud is like applying a splashy treble filter on any sound / song you hear LOL it's pretty bad even Toneking Viridis treble sounded tame compared to it!) Also yeah some earbuds and even IEMs can have better soundstage and sound quality sometimes (rarely) than some openback fullsized headphones.The K702 are a LOT bright, but in a different way than a lot of earbuds (and to some extent headphones as well) in that they do have a boosted treble (mid and upper), but they are not boosted in the presence region (2K-5K). This is where it hurts my ears and where the shouty(ness) comes from for me.... I am not (actually) treble sensitive unless there is actual sibilance or tizzyness is involved (esssssssss), not perceived but real.
I have always though in this vein. With some of the best earbuds, one can actually achieve a better sound in most cases than can be had with open back headphones (again, IMHO).
Though there ARE some advancements that are causing some IEMs to (sort of) cross that threshold. I have a set of closed back headphones, for instance, that have a MUCH wider and deeper soundstage than do the HD600 (though they aren't rivaling stuff like the OG HD800 or anything that extreme). Those are the Focal Elegia BTW. And I also have some IEMs (iBasso IT-07) that can get pretty close to some of the front-runners in their technicalities. Of course, they will never be able to completely solve the "closed" feeling you get from these head gear types though. To my ears, the Raptgo Bridge have better staging than anything I have heard, short of the HD700 (which are only beaten by the OG HD800 IMO), as another example...
Having said that last part about the Bridge, I suppose I have not A/B'ed them against the Moondrop Chaconne, or Rose Technics Maria II yet to be able to say that with 100% surety.
Even the Toneking Green Mamba is pretty good and wide sounding bright buds for 65-80€ price that definetly sounds wider than the Sennhesier HD600 for example they sound very intimate but the soundstage width is pretty narrow on them so it's not that hard to beat them on that front, however their timbre especially in the mids and overall sounds quality is very good (Sennheiser HD435 2006 model is another openback headphone with extremely narrow and small closed sounding soundstage and pretty bassy and muddy overall sound as well even the Koss Porta Pro sounds much better than it LOL!
I haven't heard the Rapto's IEMs but I've read good things about their soundstage and openess on reviews before, but yeah even the widest and most open sounding IEMs aren't that open when compared directly at similar sounding / tuned earbuds or openback headphones, also the cheap 10-15€ Superlux HD381F (white colour ) IEM / half in ears are pretty open sounding for IEMs and are tuned after the old Sony MDR-EX90LP, I've got the black HD381 versions as well but they aren't as airy sounding as the more neutral / flat tuned HD381F, here's a nice post / Review of the Superlux HD381 earphones series from furyossa:
https://www.head-fi.org/threads/superlux-hd381-series-mini-review-comparisons.493052/post-16057748.