I tend to use my HHSS in layers depending on the temp. If it is just a little cool out, in the high 50's, I'll just use the bottom layer. A little cooler, and I use the pad and bottom layer. A little cooler, and I use the top cover, pad, and bottom layer. When it hits the low 30's, I use top cover, pad, bottom layer, and emergency blanket.
I've slept down into the low teens comfortably with the HHSS. I'm not sure if the layer system is the best way to go about it but it works for me.
anyone else do this?