Halcyon vs MEST
I used the same Norne Audio Silvergard S3 cable/foam tip combo on both iems for a quick test.
Filters on the Halcyon are the green bottoms with blue top. This combination was the closest I could get to the MEST's sound sig.
Not being wordy and straight to the point, everything on the MEST is much cleaner and better detailed than the Halcyon. When I listened to the MEST and went to the Halcyon, the MEST made the Halcyon sound muddy with an added veil. It's crazy as this is something with the Halcyon I've never noticed!!.
I haven't heard the Halcyon, but I own the MEST.
As has been alluded to by others, you're not always going to be able to get the clearest picture of a comparison between two IEM's by simply A/B testing them immediately one after another. This is a technique I do use, for sure, but experience has shown me that it's also necessary to enjoy extended listening sessions (ideally whilst making notes) for each IEM in isolation.
To illustrate this, when I switch from the MEST directly to my Empire Ears Phantom, I feel the Phantom sounds very veiled and muddy.
However, if I have an extended listening session with the Phantom, that veiled/muddy feeling will disappear in 10mins or so as my brain adapts to the different sound signature.
I may not even notice any veiled feeling if I listen to the Phantom having not listened to any other IEMs that day.
Similarly, when I go from Phantom directly to the MEST, I feel the MEST sounds a bit thin and lacking in body and richness.
Again, a lengthier listening session will disabuse me of this notion
And the key point here is that, for me at least, both of these IEMs are extremely good, just in contrasting ways.
The MEST is outstanding I feel, but I'd certainly encourage you to give the Halcyon some extended listening, away from the MEST to help you get a better feel for its strengths and weaknesses. Anyway, hope this is helpful to you, or others