PSR is poor (or supply rail induced distortion) so the background noise (from ripple) increases with power and this makes THD+N constant with power. I have similar measurements in my prototype archives.I has predictably Higher noise and distortion at any frequencies. The fact that SINAD stays put as voltage increase is a bit of a concern no? It requires more digging. Does it means that the noise floor increase at the same rate as the volume? That would be a bit odd and not a good thing, If not it would mean that distortion start to increase as soon as you increase level, from zero. Also odd.