The weird sound of Nuos and Zero:2s are almost surely resonances due to shallow insertions. They make this very tiny shells to be able to produce them for cheap, and they just don't work for big guys like me. You can read about this in the Zero:2 thread. For a lot of people they sound really fatiguing and bad because of the insertion issue, I even added a video there by Resolve explaining why this happens. All the IEMs that I like fit way deeper. If I have to define exactly what
plastic sound is, it would be something like "resonances inside the ear canal that are extremely fatiguing due to shallow insertion". Those resonances make the IEMs sound like cheap, plastic speakers, like the ones that were omnipresent in the 90s, thus why I choose those words (English is also not my first language, I struggle a lot with nuance).
I think that I get driver flew with the EW200s for the same reason. They are so tiny that the whole shell gets inside my ear, completely blocking the venting hole, which is near the top of the nozzle.
MP145s are the cheapest that I have bought that I enjoy. In fact, I not only enjoy them, but they are my favorite. Just excellent. The Best Bang for my Buck for sure.
The Kefine Delci should arrive on April 15. Maybe they will dethrone MP145s as "Best Bang for my Buck", but little shells won't do it for any tall person, I'm afraid.