Everything you write makes sense, but then again, I am striving for direct applicability.The noise floor is the distance from the peak level of the signal to the noise at the quietest level. With 16/44.1 that is a distance of about 90dB. That is a bit into overkill though.
DAC filters wouldn’t have any impact on audible sound if they were properly used. I suppose you could deliberately set them so they degraded the sound, but that isn’t the intent. I don’t see any point to that feature on a consumer playback machine.
In the absence of well-defined specs, to which manufacturers will feel to oblige (and they rather do their best to be distinct and special, understandably), I can only see "transparency" useful as a purely hypothetical concept.
If, for instance, I found some USB DAC more vivid (clear, etc) than a simple Apple one - and it is as clear as scotch vs. vodka - how would I find even better one (for me) based on specs?