David,
I have had a Marmot 15 down bag for at least 10 years. Excellent bag. 2 Years ago bought a Marmot 0 degree bag. Havent used in zero temps but was very comfortable a 9 degrees.
The 15 degree bag has worn well.
IMOP Marmot is a good choice.
Lefty
Superb products, high performing.
Rapid loft achieved after unpacking.
I spray the outside of sleeping bags with DWR for added "mist/snow proofing" and keeps them cleaner.
Add a Cocoon Mummy Liner (or equivalent) for 5-8 degrees added warmth factor as well as keeping inside of bag clean.