I would go with a silk liner.
Be sure your thermals and socks are polyesther or poly-cotton blend. They should be no more than 30% cotton.
Also, take caution with that down bag. On a 6-day hike, if it gets wet, you won't be able to dry it in time for the next night you'll need it. Down compresses well, and is unbeatable for warmth - when dry.
It is up to you to take that risk, I'm just throwing that out there for you. I've seen people pack two bags, one down, the other poly - and both meant for warmer weather, thus allowing both to pack down more easily, and yet together offer better insulation. That's too complicated for my blood, but some people like that arrangement.
You mentioned the temps can range from 9C to -3C, which means, you could be crossing the melting point. When temps stay really low, your risk of getting wet is lessened; when temps are really high, your chances increase - but then you don't need the down. When you are at the melting point thresholds, snow turns to wet, whereas if it was really cold, snow just blows off.