At @prfallon69’s request, here are some high-level thoughts on the U18t, and per that request some commentary vs. CA Andromeda and Shure SE846 … ahead of a full review …
The U18t is the most tonally neutral (I’d say it’s actually neutral) of the four current universal-fit 64 Audio IEMs - especially with the M20 Apex module installed.
I’d generally consider the Andromeda to be largely neutral, also, provided those are driven out of a source with about a 1.5-2 ohm output-impedance. If your source for the Andromeda has a lower OI than that, then they’ll exhibit an elevated bass response - which won’t be the case with the U18t. The SE846, even with the “bright” filter, will also have a somewhat bassier (quantity) presentation than the U18t (I could never get the bass on the SE846 dialed down far enough on the SE846, which is why they became first a “fun” IEM and the ultimately I sold them). Using the M15 module in the U18t will bring the bottom end up to a level more satisfying for those that like a fuller bass response.
One of the first things you’re likely to notice, especially in comparison to the SE846, is the open and very spacious nature of the sound. That’ll be even more pronounced with the M15 Apex module installed.
The top end is extremely smooth and extended, and exhibits a similar type of “sparkle” and sense of “air” to that I found on the Andromeda. No sibilance or grain. Yet it is more than capable of the necessary bite for brass etc.
Detail/resolution, across the spectrum, is as good as I’ve heard in the IEM world and is ahead of both the CA and Shure models. Bass-resolution/texture is still the forté of, well, the tia Fourté (and Trió), but it is excellent nonetheless and while not something that sticks out the way it does with the SE846, shines in relative terms compared to either Shure or Andromeda with more control and articulation.
Coherence is well ahead of the SE846, which I always felt had the bass notes somehow delayed beyond the leading edges of higher frequency components in the music. I never measured them to see if that was actually happening, but as impactful as the Shure’s bass is - it always felt a bit “slow” or “delayed”.
The even-handed, neutral, presentation means that the microscope-like detail rendering of the mids and highs are easy to hear directly, and in comparison to other IEMs. They are not overpowered by the bottom end nor lost to excessive treble energy. The only IEMs I’ve heard that are as good, balance wise, in this regard, are the EE Zeus XR (Adel) in “R” (reference) mode - but the U18t still manage to out resolve them.
I find the Apex system (or similar, like Adel) to be pretty much essential for long-term listening comfort for me now. I’ll be interested to see how you find it.
I think the U18t are excellent and, unless you want a very specific non-neutral signature are very hard not to like.
I’m currently trying to decide on a custom IEM. Since I have the tia Fourté already, I am thinking either A18t (which is essentially the U18t in CIEM form) or was dabbling with the idea of the N8 (warmer/darker/bassier). If I do the A18t they’d replace my current reference monitor (EE Zeus XRA), which is looking more and more likely.