The C16 is both source and power output sensitive. That's my observation with my various amps at least.
On my Sabaj Da3, its treble is pretty smooth on balanced output until I push it past a certain loudness threshold. Then, sibilance kicks in and high register violins start sounding a little harsh.
But when I switch to my E19 and HPA-3B amps, at subjectively equal loudness settings, i don't detect the same issues. But they manifest again if I push the volumes even higher.
On the same amp, the ZS7 plays louder than the C16 for me. So what you're encountering might be what I described above. When you adjust the volume settings louder while using the C16. Thus causing some sort of distortion from less than optimum damping of its drivers at higher output levels. Or, the distortion of the amp in the higher frequencies get sufficiently high to register on the C16 at higher power outputs.
Note that many amps will have a higher distortion in the higher frequencies as power output increases.
Try a different amp or try listening to the c16 at a lower loudness level and see if you get the same issues.