What led you to repeat bits in the 9->24 example instead of rescale the values? i.e. '(n * 255) / 0b111'? Repeating bits seems like it could potentially give an uneven curve (though not significantly so), and I would have expected the DAC to roughly do the equivalent of the divide.

Historically as a graphics programmer I always see rescaling, potentially with a response curve like sRGB on the back end.