I agree. And can/do such things within my limited experience.
Closed backs didn’t do it for my wife. However, IEMs did.
I haven’t heard the monarch. I have heard (and love) the dunu zen. I also heard the Audeze Euclid. And, the LCD-XC is in my top 3 headphones with the LSA HP-2 Ultra and Drop Ether CX. All of which are very different.
From a sound isolation from others perspective, my answer is short: Get the IEM you want.
From a sound quality perspective, you can find what you love in IEM or Over Ear. Since I haven’t heard the monarch, I will talk about the Zen vs the XC.
For me, these two are more similar than they are different. Both hold similar values and have similar flaws to my preferences. My biggest argument for the Zen would be that the XC isn’t double the price better (that may apply to the monarch as well). The zen has near planar speed while still maintaining some of the note weight and slam that a DD driver can have. Before I found my HP-2, the Zen was hands down my favorite headphone of any type. It has fantastic articulation, but bass quality, quantity and texture that I personally don’t even get out of planar over ears.
The XC, certainly a step up in resolution and detail. But it didn’t really beat the Zen in the bass, which was a surprise to me. It was clearer but seemed to lack some texture and slam was only present at very high volumes. As a very competent over ear, it definitely is a larger soundstage. It feels cavernous. The zen feels like a super coherent coccoon. I wouldn’t cry if that was the only headphone I could ever listen to again in my life.
Now, I directly compared the Zen to the Euclid in a post on here. And, for me, the things the Zen did well made it better than the euclid. But, the euclid, at twice the price is a touch better in things not relating to slam or note weight. And it is very similar to the XC. So if you wanted the XC in an IEM, that is where I would go. It was also a touch more relax in the mids, which I am sensitive in 1-2k. And the FR extends in both ways a bit more, which can make some music sound more “complete”.
So, IEM for the isolation. I joked that if I ever wanted to buy an open back headphone again, I would buy the LCD-XC. To me, closed backs are pointless for isolating your music from others. At least my wife can’t sit in the same room and deal with it. IEMs? Work great. Even the Zen which are vented and leak more sound than others.
Happy to answer questions.