No worries Kevin. Like I mentioned, probably not helpful in choosing a down bag but more info is better than less... sometimes.
The BA bag I use most often is rated at 15 degrees. I also have a 30 degree bag and a zero degree bag. The 30 is an oversized mummy and I use it more like a quilt with a foot box in warmer weather than a sleeping bag. I have looked into the over quilt craze. The 30 degree bag works fine for cool nights. In my experience, using a heavy quilt for cold nights wouldn't work for me. Every time I roll over (which is a lot I guess) I flop the cover off and end up having to re-tuck it around me and re-warm the inside. That's OK for cool nights... it would not be good for cold nights.
The zero bag is tight to me (well... maybe tight on me since I'm a bigger guy). I have a Gore Tex bivy sack that the 30 and the zero will work in but since I sleep hot I always get some condensation. The zero bag and the bivy usually live in a tub in the back seat of my truck during the winter and on trips to the mountains as an emergency or back-up bag.
I think your tent or shelter choice should inform your bag choice as well. If you go ultra-ultralight with a super light tarp and use trekking poles for the supports, your bag may need an outside shell just to keep it from getting soggy on the ground. If you do a tipi tent with a stove, a different option might be better.
On back country hunts where it all goes in on my back I have a Mountainsmith 3-season tent. It was sold as a 2-person footprint but it is definitely a 1-Keith size. It has a nice vestibule where I can keep gear (and MOST importantly boots!) inside at night. It's small enough to pack but large enough on one end (wedge-shaped) to sit up and pull on clothes in. I have been using it for a couple of years now and really like it. Again, I am sacrificing increased weight for more comfort but it is worth it to me. One advantage to having a strong back and a weak mind I guess.
I have way more gear than I can use on one trip. My wife claims that when she is gone, she thinks I lay it all out and roll around in it. Why else would I need so much stuff? Sleeping bags, tents, stoves, cook kits, etcetera, etcetera, etcetera. I suppose I could be spending the money on worse "life choices".
OkKeith