It's both, because I hear little bells in the background even at low volume. An IEM is very sensitive so needs very little power, the amplifier will perform worse as it needs to output more power.
In fact when I use over ears, it sounds better because I increase the device volume which increases the input voltage
Anyway, the $9 Apple dongle blows my phone's 3.5mm jack out of the water. My tablet and desktop have the same issue, but when I connect the same devices to my ancient laptop they sound perfect.
The point is the 3.5mm jack actually gets me worse sound quality because my phone doesn't output audio to usb, so I only use it with my TWS. Which, by the way, also sound like crap in the same game, but it might just be Bluetooth issues