Cloud Nine

I've done this, works pretty well.. I had a down bag with a comfort rating of 3 degrees and an alpkit down quilt and got a few degrees below zero. I generally sleep cold. Edit to add: on a warm summer's night the down quilt by itself is lovely.

Alpkit down sleeping bag. Best bet for down bags, used mine 3 years and has been perfect

I'll just mention that my snugpak 'winter' bag is rated to -10C, but there's no way I'd use it in those temperatures. Take the rating with a pinch of salt. My Alpkit down bag (-7C comfort rating) is way warmer than the Snugpak. I don't know about the arctic bag, I have the modular system. And yes, get a decent mat.