Which Big Agnes bag did you get? I've got a 6-year-old Storm King, which is supposedly rated to 0 degrees. Supposedly. I've got an equally old Big Agnes inflatible pad which has fibers of synthetic insulation inside.
Pad Here It works great, but it really cold temps, I double up with a 3/4 length z-lite sleeping pad from my head to my knees, and then I put my backpack under my feet. Works like a charm. It was quite cold at Meysan Lake this past weekend, and I was directly on the snow. This combo kept my warmth from escaping down. I will say, although I like the extra room afforded by my Big Agnes, I think I should have got the model with the slimmer cut so there is less room for my feet to heat up. That's really the only spot I have trouble.
Bottom Line: a big heavy expensive pad just for winter use is not necessary. Use a 3 season pad coupled with a light short closed cell pad, and your pack. Good to go. That is, unless you like carrying tons of weight and spending lots of dough =)
