I use Unicorn Fibre Wash and Rinse. Man o man. It can really clean the yarn, without being harsh! I highly recommend it.
It really did a great job of softening the Tahki Donegal Tweed, and also removed the excess dye, which I think contributed to the scratchy feel of the tweed. You wouldn't visually notice the excess dye being gone ...but the feel
of the sweater was incredible after the wash and rinse. (my daughter's Central Park Hoodie)
Saw it advertised in a magazine I subscribe to: "Wild Fibers".
I haven't used anything else since!