In the late 1960s, I did my first horseback hunt -- a DIY in the 10K + ft. mountains near Vallecito Lake, Colo. So I bought a full-sized Slumberjack down sleeping bag rated for -10 below. I still own it today. It had a heavy, durable cover, but eventually, the zipper went kaput.
For many hunts, I just used a mummy bag filled with 3# of Quallofil, which worked just fine down to about 20 degs. But when I would go on a hunt where I knew the temps would be lower than that, I would put the mummy bag inside the down one & roll them up together. No matter the temp, that combo kept me quite toasty even with the busted zipper on the outer bag.
Ironically, in all the years, I'ver never owned a down-filled outer garment. My go-to jacket over the past 40 yrs. for cold weather was the inner lining from a Bob Allen coat. It was reversible with camo on one side & orange on the other, & like the mummy bag, was filled with Quallofil. For cold weather, I generally wore the med. or heavy weight Thermax long-johns, a wool shirt & the jacket.
I also have a complete set of heavy wool stuff from Woolrich. It's a shirt/jack, pants, & vest, as seen below on my NZ hunt for tahr..