why do you think "This difference gets lost in the THD already at the DAC "? Do you have numbers to back it up? What's the noise floor of DAC? What's the noise floor of an output stage? Do you have the number?
high dynamic range is not about the lowest volume you can hear, it's about the voltage resolution between this sampling point and the next. Base on your assumption, we can all see black whether we use 16bit RGB color or 24bit RGB color, what's the point of using 24bit RGB?
Many years of building audio equipment (in particular analog synthesizers), and equally many years of being meticulously anal with getting the best components for my circuits, reading specifications of down to every single op-amp I've ever employed, is why I think so.
I am not saying that there aren't any DACs on the planet that can't handle five millionths of a volt, but I am saying that five millionths of a volt isn't surviving through the particular DACs and the rest of the electronics used in your PC/living room hi-fi audio equipment.