You can forget about soundstage aspirations in IEM's
Soundstage is a barely defined attribute, with folks attempting to define it in some sort of way. The current fledgling understanding is basically your ear shape will have an effect on the outer ear (the pinna) and how sound interacts with it. Since IEM's are inserted into the ear canal, this sensation is all but lost. But this is only part of the physical phenomena.
Many will praise the HD800 for "wide soundstage". Yeah well naturally considering the size of the cavity (and other things I will get to shortly). When you have such a massive space, and drivers far removed from the ear, you can essentially get better "soundstage" than many headphones. But also it helps HD800's have elevated treble spikes that contributes to also "air", but MOST importantly the third factor that has gained them this reputation is the comparatively low THD specs of the headphone itself.
As for other aspects and my personal belief about soundstage.. It's mostly comprised of quality of recording (not just using great gear, but properly spaced and proper work from the recording engineer), setting of recording, editing/mastering/post processing. I would wager my life that more people than not, can take any modern headphone (or IEM), and listen to a recording of Amber Rubarth's "Sessions from the 17th Ward" album, and will praise their feeling of soundstage. This has nothing to do with the listening device at this point, and everything to do with the qualify of recording (binaural) and setting (church hall). I highly urge you to listen half the tracks if you find the time, and come back and see if you still feel your IEM's can't produce any soundstage.
The last portion is the editing/post-processing involved. The first of which, being channel panning (self explanitory). The second, DSP and audio effects can be added to induce the simulation of soundstage. You can literally take the most "un-soundstagey" audio sample (that being a mono recording) and simply adding in some reverb effects, and you instantly have more "soundstage". This is the sort of thing that is being pioneered by Virtual Reality companies currently (collecting sound samples, and coming up with new ways of emulating what people sometimes describe as soundstage).
In conclusion, because no one actually has a firm reproducable grasp of soundstage in the hardware realm we can't in good faith make solid recomendations when people seek this sort of factor from a hardware perspective. My firm deductive reasoning would be just having high quality drivers with great performance, and a large enough cavity where your ears don't touch any parts inside as much as you can. Outside of this, I haven't seen any sort of research to suggest any parralell with what some people have claimed are "good soundstaging headphones" and such. Opposite to an example I raised earlier.. you could in fact get HD800's and it wouldn't matter much for soundstage if you turn on some of the highly compressed modern Pop genre atrocities we call "music". You'll see your soundstage won't scale much regardless how good your headphones are.
So.. in second conclusion. Forget about searching for "soundstaging IEM's" from recommendations, instead try it out yourself, and pray you will placebo yourself into thinking you hear soundstage from an IEM. If you want more feeling of soundstage.. seek out well recorded music, and if you MUST get IEM's, then seek out ones with low THD, and good FR consistency. Outside of that, it's up to you to DSP your own soundstage somehow.
As a resource on this,
you can check out this site, as they're the only ones seemingly willing to seriously share and seriously persue this phenomena in some way.
