It's scientific that you can push an amp into clipping if it doesn't have the current, although it more typically happens with lower impedance headphones than higher ones.

Volume controls voltage and with low impedance headphones you usually can turn the volume up higher than the amp's capability to supply current, if you do this you do get clipping and a huge spike in distortion.

You can see this in the 16Ω measurements for the KA3 unbalanced, and the 32Ω as well with the balanced. It's not apparent in the 68Ω or 300Ω though, that goes to max voltage without clipping, although distortion does start to rise again above 20mW (not to the level it would likely be audible).

https://www.l7audiolab.com/f/fiio-ka3/