I always use a pad for under my legs...depending on how low the temps go, I double over the pad. This is the winter version of the Yeti...in the spring/summer/fall here is the US, you could just put extra clothes or even your pack under your legs if your using the 3 season Yeti).
Personally, even though I have a double BB, I put the leg pad inside with me(not between the layers) for 2 reasons...
1. I like being able to move the pad around as I move my legs...easier to do inside with you
2. Since your legs have very little weight (compared to your shoulders/butt) I don't even feel the pad (like you do under your butt).
So to answer your question, if your body weight allows a single BB, do that. Your pad/Yeti combo works with a single layer.
Experiment with what feels good to you!
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