Sorry to hear that.I contacted to the hifi-express store, where I bought from the Smsl sp200 about the hiss noise with earphone, especially with low impedance earphones like the kz as10 that has impedance of 14 ohms and after 4 days and two messages they replied,
"14Ω< 16Ω
The headset is too sensitive to hear the current sound of the thx module,not suggest to connect it with KZ AS10."
This is really upsetting response, the funny part is that before I bought the AMP from them I ask them if the AMP work with earphone that has minimum impedance of 8 and they said "yes,it can" .After I told them that they told me the I need to get a "Impedance noise reduction rod " which is a 3.5mm Impedance Adapter . I ask them for a replacement or a refund as I got a defective AMP but they still did not reply.
However, 14Ω impedance has nothing to do with the sensitivity of the headphones. We could have 14Ω earphones with a sensitivity of 115dB/V and 32Ω headphones with a sensitivity of 125dB/V.
Regarding your KZ AS10, per manufacturer specs, these IEMs have 14Ohms impedance, 105dB/mW or 125.5dB/V sensitivity (indeed, very high!), although per https://reference-audio-analyzer.pro/en/report/hp/kz-as10.php your headphones are having an impedance of about 17Ohms and a sensitivity of over 132dB/V (extremely high, you might need to purchase Geshelli Archer 2 to drive them without background noise).