Gee, Bill,
Is this an interesting question!!
Truth is, what audiophiles often rate as transparent is anything but technically.
They will say, and subjectively I agree, that the single ended triode, a really good one, is the most transparent thing out there. Yet, when you measure them, you see that they have about 2% THD, most of it H2, H3 and H4. This is in fact very coloured, but it's clearly what many - not all - really like.
This points up that whatever THD is measuring, it's likely not transparency.
Sorry, just muddied the waters....
Hugh