For measurements, I am using the same in-ear coupler as my friend on Headflux.de, the IEC 60318-4. I am posting raw measurements, which means I have not compensated them to any target. The curves are smoothed according to psychoacoustics in REW.
Please keep in mind that the 64 Audio tia driver is very dependent of the placement on the coupler. Any channel imbalance might be due to a slightly off angle.
Frequency Response: 64 Audio U12t
This is a very attractive frequency response curve. Notice how the bottom/ground is flat and the bass is focused on the sub-bass, reducing bloat and warmth. The pinnae gain is not fully compensated and falls short a few db in the area of 2.5-4 kHz by neutral standards, but this does help to reduce fatigue. The treble is smooth and has superb extension.
Frequency Response: 64 Audio U18t
Surprisingly, the SPL drops in the lowest octaves. There is mild bleeding into the midrange which gives a subtle warmness. The treble looks absolutely worthy of a flagship, even though it has a slightly bright tilt. As mentioned earlier, the channel imbalance in the treble might be because of positioning on the IEC, even though I re-adjusted the IEM a few times.
How the U12t compares to U18t
As mentioned in my comparison, the U12t has less warmth but more emphasis on sub-bass. Most of the midrange is similar. The U18t has a bit stronger highs, though.
The Difference Between Apex M15 and M20
The U12t and U18t come shipped with both apex modules. All other CIEMs are equipped with the M20 per default. As I mentioned in my listening impressions, I preferred the M15 for a leaner and more spacious sound. I was under the impression that the M20 module would add a bit more bass than the comparison shows. I have compared several measurements and have always received the same results. Objectively, the difference between apex modules is almost negligible.
Frequency Response: 64 Audio Fourté Noir
What a surprise! Notice that unexpected 1k dip. It puzzles me how the sound can be so amazing with a non-reference SPL curve like this. No surprises with the bass, which seems perfectly tuned. The treble, though not the unequaled perfection of the U18t, features a curve that definitely classifies as top-tier. Back to the midrange: There must be some unexpected psycho-acoustic magic going on that I cannot explain. Looks weird, sounds great!