Holy crap that’s concerning…I am using the same "super slow" filter. But it is important to configure the source sampling to the highest possible to avoid the noise to be on the audible range. I use it with software upsampling to 32/384khz and it is the best sounding option for me. It mitigates the sibilant sound of this DAC.
Nevertheless, my K9 Pro AKM has some kind of ultrasonic noise that makes my ears hurt after a while. I can't hear anything but I now there's something in there, because after about half hour of using it I end mid-deaf. None of my other DACs produces that problem on my ears. And it is volume level independent. Very weird issue.
Actually I don’t find it sibilant at all. The fact that it’s not volume related is also pretty weird.
What about if you listen at low levels? Like 60db?