I have personally found that with my bag (Marmot Pinnacle)the liner which is Sea to Summit's alleged 25 degree liner, I was much warmer than with the bag itself.
Since I don't "feel" in Fahrenheit or Celsius I can only say that I was much warmer and cozier while a light dusting of snow was going on outside my tent's wall.
With my quilt, I found the liner to be much less effective so I don't bother with it.
I almost always sleep with merino long sleeve and full length leggings along with some synthetic fleece socks.
Now what I did a few times this summer was to use a homemade fleece bag with the liner and it was fine for a "chilly" night. Really light but puffier than my other setups.
Bob
