I don't have the ASX, but that is a common issue in highly sensitive multi driver IEMs, they do pick up hiss. The ASX has a reported sensitivity of 106 dB/mW, that would be considered quite high sensitivity and I wouldn't be surprised that hiss occurs. It is not the impedance value but the high sensitivity that may cause hiss.
There's a few ways to remove the hiss:
1) use an in line volume controller
2) use an impedance adapter
3) use a high quality DAC/AMP as a source. I'm haven't used the SMSL AD18 FDA amp but it seems to be quite a basic amp and doesn't have the best reviews for sound quality and background noise on ASMR:
https://www.audiosciencereview.com/...w-and-measurements-of-smsl-ad18-dac-amp.7325/
There's some cheap and good DAC/AMP dongles u can try, that ain't expensive, like the Tempotec Sonata HD Pro, about $30 USD, very clean and neutralish sound, very transparent with no hiss even on very high sensitivity IEMs. It doesn't get warm too and has volume controller on it. Very good measurements and low output impedance. That's my go to when I go out, I just plug it into my smartphone when I want zero hiss on my multi driver high sensitivity type IEMs.