NEMO - Disco™ Men's Endless Promise® Down Sleeping Bag (15°F/-9°C)

Nemo Equipment men's and women's zip together and the Disco line (I think that's the one that's down, not synthetic) is very compressible. They're also spoon shaped for side sleepers. I have one that I'm not longer using since I went to quilt life. I have zipped the men's Disco with a women's Forte with zero issues at all! It's this one (https://www.rei.com/product/163032/nemo-disco-15-sleeping-bag-mens (Torch/Stormy Night) and if you're interested in buying, I'd be happy to list it on r/geartrade for you (to go through a slightly more vetted process). EDIT: added more info

Im a side sleeper and a turner at 6'3" and 230 pounds. The quilt is great. However, if it is so cold you want an enclosed sleeping bag, I personally carry the Nemo Disco 15 large. I open this and use it like a quilt at times. https://www.rei.com/product/251886/nemo-disco-15-endless-promise-down-sleeping-bag-mens

I'm a 230 lbs 6' 3" guy and this is a great bag for me. Used in 19F and it was warm. I am a side sleeper so the ability to turn helps a lot.

The new version of the Disco 15 Men's Regular has an advertised compressed volume of 6.4 liters. The Disco 15 Women's Regular is listed at 8.8 liters. So a little over 15 liters combined.

My NEMO Disco 15 (Mens long) compresses to about 9" X 11"

I have the nemo disco 15 in the big boy size.  If you get cold easily,  I'd recommend the little bit of extra bulk and weight for the 15. The spoon shape is much nicer than a mummy bag. 

I have the disco men's long in -9C and used it in the rockies on a trip we got snowed on with a good sleeping pad. I would say it's a little cold for late fall.  Or the one I have is a bit underfilled maybe. I also find it a bit tight but my shoulders are about 24" across so I'm pretty big. Sea to summit ascent II looks to be an upgrade warmth wise and has a better zipper for venting.

Love my Nemo bags and always recommend them. Great product, great customer service and repair, and good price. And they make bags that are specifically cut to minimize dead space but accommodate side sleeping. https://www.rei.com/product/228442/nemo-riff-15-endless-promise-down-sleeping-bag-mens A few other things to keep in mind: - A great bag is useless without a pad. Make sure you factor that into your investment if you haven’t already. - A 20 degree bag might keep you safe in 20 degree weather, but you won’t be happy about it. I want a 0 degree bag if camping in anything below freezing. (But I am a small-ish woman who runs cold.)

I have had a nemo disco 15 bag, two EE and now a Zenbivy 10 degree light bed with half sheet. The Zenbivy is by far warmer than EE and to the moon and back more comfortable than EE and the Disco. I was skeptical of Zenbivy but it is a quality product. I use a Ultra 3R exped pad for three season and a 5R in cold season.

Get a disco 15 from Nemo. Only use kelty in the summers.

I have a disco 15 and love it. Alternatively you can get a fleece bag liner to add some warmth that way instead of a whole new bag.