Disclaimer: I haven't (purposefully) felted anything before...
I haven't knitted with WOTA, but it looks like it's a plied yarn. The Knitpicks website says it's nice and soft.
Lopi, which I have knitted with, is not plied, and I wouldn't exactly describe it as soft, either! Its scratchiness is what causes it to felt so easily -- you can practically see it felting right on your needles. And it is warm, warm, warm.
Sorry, that's probably not much help. So, felters, is there any difference between plied and single yarns when felting?