I'll make enquiries, though, just to satisfy curiosity!
Okay, just over 2 months later (don't ask...) I have
finally got an answer. But hey, it's come via an intermediary from somebody who is known in some circles as 'the big guy' (although apparently he isn't that big at all - go figure
)... Anyway:
The automatic should be doing Triangular non-shaped dither (this adds the fewest bits of noise to the signal, but anything less would introduce artifacts in the bit reduction). But since it is non-shaped, the hiss level will sound louder than most of the noise-shaped curves. All other noise shaping curves ‘color’ the background noise in some way, and depending on the source music, some curves will sound better than others.
This is the present situation, at any rate.