Already have a 40 degre quilt, what to get for fall and winter?

Get a 20-ish degree bag for autumn, and bring both bag and quilt for winter.

I can't speak for them myself, because I haven't gotten mine yet, but the Enlightened Equipment Revelation X quilts come on high recommendation from everyone I talk to. I plan on using a 0 degree Rev X quilt with a warm pad combo in winter, but haven't tested it.

Zpacks (I hear) makes nice bags, and even lighter. However, they are roughly $100-$200 more expensive, in my opinion not worth it for a couple ounces saved.

I'll soon be getting my UL sleeping bags in the mail (Rev X for winter, Golite Adrenaline for summer), so I'll be able to speak from experience rather than other's recommendation.

I have a 20 degree Zpacks hybrid quilt bag w/2 ounces of overfill that for me is good to about 15 degrees. Joe has now a 10 degree bag. I have a couple synthetic quilts Tim Marshall made me 3 and 4 years ago. Both make nice quilts..

Definitely not a quilt, especially without a bivy.

20F en-rated bag … youll be good to ~0-10F

I'd push the 40F quilt to 30F with warm clothing/smart site selection and then grab a 15F bag for the winter stuff.

I agree with people above.

Use high loft clothing to extend current quilt to just below freezing (when you'll be glad with the warm clothes anyway), use a bivy if in a drafty shelter like a tarp.

Then use a 20 degree mummy bag with the warm clothes down to ~0F, then layer your summer quilt over all that for real cold winter days.

Dan and Tjaard have the right idea :)