Anyways. This fiber is very, very soft (hence I wouldn't make socks using it!!) and when I tried to spin it, it was extremely difficult to draft. It wasn't felted, but certainly seemed to love to stick together. I tried pre-drafting (something I normally never find necessary with commercially combed tops) and even then, could only produce lumpy singles. In desperation I turned to the Ravelry forums and asked for help, where the prompt suggestion was to turn the braid around and spin from the other end.
Wow. What a difference! This was still not the easiest of fibers to spin, but once I started drafting in what turned out to be the "natural direction" of the fibers, it was much, much easier and I was able to produce non-lumpy yarn.
[DK weight handspun, chain-plied, D'Arles Merino, super soft!]
In retrospect, this isn't surprising, since wool has little scales that are responsible for the grippiness that makes it felt (and also repel water and dirt). The more scales, the more the stuff will felt. The modern "superwash" process does something to the scales (abrades them or fills them in) to prevent felting, so I'd imagine superwash fiber to have very little directionality. If you're careful, you can tell the directionality by petting - like you'd do with a cat - combed fiber (carded roving is jumbled, so not directional), but this was pretty subtle on the commercial top I had.
[electron microscope image of wool fibers, image from Vogue Knitting (1989 edition)]
I've since tried to discover which other breeds might be prone to this directionality. I've read that longwools can be - BFL, Wensleydale - but I've not encountered directionality in my (limited) spinning of these, so far.
But at least now I know what to do, the next time I encounter this problem!