Yes, best is down on top and just a flocked sheet on the bottom, with a pocket in it for a NeoAir. Now that's heaven. There's no reason to sleep on top of down. Our rig was made by Feathered Friends. I think Big Agnes has similar if you ask. Thus, there is a zipper down both sides of the bag, which you can use or not use as much as you want, according to temperature. This system is light enough that one can make a good case for buying more bag than one needs. Our rig is a double.

Re the complaints about a down bag picking up moisture: one of my favorite things about a down bag is that is self-adjusts to current temps. IOW, if your bag is too warm you'll sweat and your sweat will condense in the down and the bag will become a less warm bag, so you stop sweating and sleep more comfortably. Conversely, if it cools off you'll dry out the accumulated sweat and your bag will become warmer. I have frequently dried relatively large quantities of wet synthetic garments and wool socks in my down bag and had them be almost dry in the morning, with no discomfort at all. Works like a charm, due the permeability of the down and ripstop nylon - it passes moisture through it very easily.