In my experience good iems under 250-300 usd compete and/or beat similarly priced headphones. But in higher price brackets headphones have the advantage. In my taste at least, your milage may differ.

I can't speak for the very TOTL but i can speak for the 300€ range with the Blessing 2, i think the best headphone "competitor" would be the HD600, and it's safe to say that i use the Blessing 2 rarely enough to the point i sometimes forget i even have those while i use the HD600s on a daily basis, IEMs really aren't all that great compared to headphones, their only real advantage is size (at least in my opinion)

Have an HD600 and a Moondrop kato(sounds better stock), if the blessing sounds worse than the sennheiser then that's very disappointing.

IEMs are so different than headphones. Like headphones are so different than speakers.The experience I mean.

I think it's like comparing cars and bikes.

IMO, the only thing I can say is that disregarding the experience, and these days, cheap IEMs compete easily with midrange headphones.

Speakers are beating both, but in a modern environment if you’re not living in your own home, apartment isn’t best environment for speakers That said- iems are the convenient way to audio

>Is it only the sound stage or are the other differences?

The difference is the sensation of the sound around your ears (especially the bass). You may think that this is a minor thing, but it's not. Yes, this affect soundstage sensation.

The only other difference lies in the frequency response of the two devices. In-ear monitors (IEMs) have better isolation and do not interact with the outer ear, allowing them to deliver a good volume with "less effort". This makes it much easier to create IEMs with excellent sound quality and considerably low harmonic distortion, allowing for easy equalization.

Some IEMs with multiple BAs have slightly higher harmonic distortion. This type of driver is mostly used in high-end IEMs, while headphones (IEMs) with dynamic drivers have considerably lower harmonic distortion, reaching values as low as ≤0.1%.

For what I own, I definitely love Timeless > HD6XX > Sundara > Aria > PR1 Pro.

If you want to compare IEMs to headphones, you should look into custom IEMs. They have some ambience that regular IEMs don't.

I’m gonna go a bit against the grain here. I’ve tried several IEMs from moondrop starfield to campfire andromeda and they’ve all sounded like shit compared to HD600s or sundaras. Basic speakers like LSR308Ps are way more enjoyable than any headphones imo, especially with a decent subwoofer. I’d go as far as to say if you have any reasonable private listening room headphones shouldn’t really be in the discussion. No headphone matches the visceral experience of well tuned room-filling bass. I’d even take my (fairly extensive) car system over any headphones I’ve ever heard 10 times out of 10. Just IMO of course.

I prefer my music to sound close to me as opposed to making it feels like a concert that you get on headphone or speakers, so I chose IEM any day. When comparing items at the same price, I very much prefer Andromeda compared to DCA Aeon 2 or Audeze LCD-2, they just sounded more pleasing and more practical to me. Of course, the soundstage is much bigger in headphones but I can feel note textures and details better in IEM.

Edit: I already got to the point where the only headphone I keep is LCD-2 for 7 sake, but I rarely use them, maybe only a few days a month. I suggest OP try both himself and find what you like better.

My only comparison in this regard would be the Audeze LCD-i4 to the LCD-5. Both the TOTL that Audeze offers. To me, the LCD-5 absolutely demolishes the i4 but it is also almost twice the price. I would say that comparing the i4 to other sub $2K headphones and it is much more of a competition but I still think even an Arya Stealth still wins out. It is worth noting that the i4 is certainly a special case when it comes to IEMs though as it is open back and actually gives real soundstage and completely bests every other IEM I have heard. So while I haven't heard the Monarch MK2, I get the feeling that the gap is even wider.

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I feel like IEMs really shine in the sub $300 category.

for multiple thousand at the minimum in an acoustically solid room? perhaps

Budget IEMs are better than TOTL headphones except for soundstage width.

I have the Blessing 2 Dusk and if there is any headphone in existence that even sounds just as good without any EQ let alone better I would be shocked.