A good 3-season down bag has what are known as 'continuous baffles (no baffles on the side seam) so you can shift the down where it's needed. On my bag I could shift nearly all the down to the top if I so desired.
(snip)
If it's cold it's much more efficient to zip the bag up and create an insulated tube to sleep in. The less air around you that needs to be warmed the easier it is to stay warm. If it's a mummy bag there's also a hood and likely some type of draft collar to help keep that heat inside.
You have a good point on the continuous baffle and manually shifting down around -- you're just the first person I've heard of who actually uses the continuous baffle to shift down around to respond to temperature changes because most people find this to be a hassle. Good for you!
On the air volume point -- you typically don't have a lot of excess air volume around you on a quilt because the quilt settles around your body (and you can tuck it around you if you like).
Also, if the air volume in a bag is really tight, by definition you're using the most confining bag that you can. Quilt lovers are lead to them partly to be less confined. You shift in the night, the quilt settles around you again, air volume issue gone, although shifting can create a little bellows effect and blow some warm air out, nothing's perfect. I've got scoliosis and so I tend to shift around a lot to deal with the back pain -- confining bags are torment to me.
On the draft tube point: what you say is certainly an issue with some quilt designs, others not so much.
Here's what I and other quilt users I know do: on a medium cold night, we tuck the quilt around our necks and it stays there, like a blanket at home. Draft tube solved. On REALLY cold nights you can duck your whole head under the quilt and just leave a little gap to breathe through. It sounds involved, but it's not -- it's natural as can be.
However, all that said, since bags work for you: that's great!
For anyone who is starting to feel a little confined or wants more flexibility with their bags, I strongly suggest giving quilts a try. I have never known anyone who tried a decent quilt who then went back to a bag.
And with that I'll stop posting on this thread because I think I've gone on too long on it. I just wanted to share the love
