I have two EE quilts and have used them below 0 above 7000' and have stayed warm. One is a 0 degree made with cuben fiber (not made any more). It is a vapor barrier. The other is a newer 20 degree. With the 20 degree I used a WM Hotsack VBL (Vapor barrier) and kept warm in single digits.

I also have a WM Antelope that I've used below 0 and I prefer the quilt.