Disclaimer:
A4s is personal unit and I bought it for myself.

Background:
My last IEM (before the A4s) is JH Audio JH5. Two BA drivers with also custom-fit. I purchase it around 2015-2016. It was a good pair IEM for me personally. The sound is laid back mid area with smooth treble and enough bass.

After 7 years, I thought it’s time to upgrade. Since I’m in Indonesia, we don’t have any freedom for CIEM brands. Local retailer only have 64audio and local CIEM builder.

Why CIEM?:
Personally, I don’t want to spend big money on common stuffs. I want my things to be as personalized for me. Custom IEM is perfectly matched for me. Also I don’t have any issues when using my JH5.

In the local retailer, I tried U4s (with M12 Apex Module) and I love it. I like the midrange, the enough bass (but bigger and better than my previous JH5) and also the sparkly treble. order A4s and I wait 3+ months to arrived. Spoiler alert: I need to wait 2 months extra to become satisfied.

Review:

Packaging:
The packaging is simpler than the universal version. It only comes with IEM unit itself, a CIEM cable (that different with universal one), M20 Apex module (-20db isolation), candy case from aluminium, silica gel, and IEM cleaner.

Build Quality:
To be honest, I disappointed with the build quality. The resin is so thin, and the IEM itself feels so lightweight. Maybe related with the APEX module so it needs chambers in there (?), I don’t know. But my cheaper JH Audio JH5 feels sturdier than pricier A4s (JH Audio resin is very good and very solid).

Sound:

Bass:
A4s comes with M20 Apex Module that have HUGEEE bass. Sub bass is slamming hard, the body is fat, and also bass decay is very good and very rich, thanks to the dynamic driver. It is not typical fast bass.

Midrange:
It is not the lushest vocal that I have ever tried in IEM. It feels right. It is not laidback, and never sibilance. It just right amount with right intensity.

Treble:
U4s in universal form is very energetic IEM. The treble sparks. A4s is smoother than U4s. It’s not as energetic as in universal form, but still extended.

Upper treble:
I feel in the upper treble is emphasized, maybe because of the tia driver. In the first try, it feels weird. After some times, the upper region feels so open.

Imaging:
One thing that I love with A4s is very good handling complex percussion. Tracks like Polyphia in Remember You Will Die handled very well. I can easily pinpoint any instruments that coming from the drivers.

Summary:
A4s is a good basshead CIEM. But (personally) I feel disappointed. I am not basshead. I do love bass, but not over-bass. AI tried U4s with M12 module and I like it (because the amount of the bass is very appropriate for me personally) and the A4s sounded so bassy.

After I got the A4s, immediately I tried the M12 module. The bass tamed down, midrange and treble more open than before. So I ordered another M12 module, and need to wait 2 months to wait. A4s with M12 is a killer IEM for me personally.