Three-season bag vs. 40 degree bag+liner

Liners don't do much maybe 5% at most and there heavy. Have you considered a quilt? A 20% quilt will come in at about 20-24 ounces and you can vent it so you don't overheat. But if you want one bag defining buy the bag or quilt rated for the lowest temps you will see

If you search old topics, you'll find that the consensus here is that the S2S Reactor is over-stated in its capabilities, and probably is only good for 5 degrees more, like a light fleece liner.
So, it won't do what you're hoping.

However, the Montbell UL Spiral Hugger Down Thermal Sheet will put 15 extra degrees in to your bag rating, albeit at a much higher cost.

Alternatively, just wear your puffy down jacket and pants inside the bag, and that could also do it.
Any way you slice it, you have to carry more fabric for the temp rating if you layer.
I layer, and accept the extra weight for the versatility of layering.

Interesting, thanks for the comments on the S2S and the rec on the Montbell. I had seen that, but hadn't looked closely but I'll do that now.

I'm glad to hear other people layer–if there's one thing I've learned from buying gear for other hobbies like cycling, it's that I'm never happy with multi-use gear, but with the layering potential in sleeping bags it seems like a different situation.

Any comments on the Western Mountaineering Highlight 35 vs. the Marmot Atom?

Edit: just realized the Montbell won't work for me, max height of 5'10".

WM Sycamore MF

If I could start over from last year, this would be the single bag I’d get. Instead I’m stuck with 2 new fraudulently temperature rated (off by at least 10 to 15 degrees F compared to their 1990s ratings) Northface bags I foolishly bought on sale before realizing that NF wasn’t the same decent company they used to be, and some old bags that weigh more but were at least honestly rated.

That wm bag is worth a look, 25 degree bag, full unzip so you can use it as a comforter in summer or warm days, very light.

In the 80s I did a lot of cycle touring, California, Norway, Europe, and I used a single I think it was 15 degree Sierra designs bag, it was good for all seasons, down is fine in summer you just unzip it.

From all my reading here, it sounds like one of the only brands out there now that you can trust the temperature ratings on is Western Mountaineering. This is sad, all the big players used to be honest and didn’t exaggerate, up til they all started going big time corporate. Hard to believe Northface was a cool local company here in the Bay Area a few decades ago, same with Sierra Designs.

How tall are you?
Theres no hood on the Montbell Thermal Sheet, so there's a little bit of fudge factor in the length. If it's an inner bag, then it doesn't have to close around your neck. Shoulder height would be fine for an inner bag. You could try it out from a place which accepts free returns if you don't like the fit.
If you are over 6 feet, it probably won't make the fit for you, and it is quite tight on the body if you are not quite slim. But it works for me and I'm 5'10 and 200 pounds, but I barely can zip it up. I don't zip it up all the way when it use it as an inner bag.

About the Western Mountaineering bags, I have one and it is very well made and high quality throughout. I have the Alpinlite 20* rated bag.

The Highlite is an awesome summer bag, and is one of their most popular. It lacks full baffles, but that's not as important in a summer bag. And it's very light at 1 pound.
But the "lite" series WM bags generally are VERY tight fit if you have any bulk on your bones. They are made for lighter weight people. If you have some bulk on you, even if you are a weightlifter or something, you might want to look at the Caribou or Megalite which are a bit larger across the upper body. They weigh a little more, but if you need room, those are available.
Also rembember that if you plan to layer another bag under them, or wear more clothes, that you need to have room for that in the bag. If the bag is nearly skin-tight, you have no room for that. So, be sure you are sensible with your fit choices.

Marmot Atom is also a nice bag, but a little heavier, and not rated quite as low. But still good. Has some room in it, similar to the WM Caribou(35*F), and similar weight as the Caribou. Marmot has a good reputation which is well-deserved, but I think the WM makes a better bag IMO. I'm biased about that, I admit, because I really like WM bags.

I also agree with the above poster about the WM Sycamore. That's a very nice roomy bag which leaves space for layering inside. It weighs a bit more at 2 pounds, but it has the advanced Microfiber shell and alot of room, and rated 10 degrees colder, so it has benefits for the weight.

My "rule of thumb" which is my personal "rating system" is to always expect at least 5 degrees less performance than the bag is rated at. Maybe I'm just a cold sleeper, but that's how I rate the bags. And if the bag is not a WM bag, you might consider rating it at 10 degrees less performance than the maker's claim. I'm serious about this, especially if you sleep cold.
"Cold sleeper" in this case means you feel cold easily when the temps drop. Not sure if everybody uses that term the same as I do, but that's what I mean by it.

And always get a good mat/pad which has enough "R" rating insulative qualities for the temps you plan to go out in. The pad/mat is responsible for about 35% of your warmth, so it is an important part of your sleep system.

Thanks for all the info!
I'm 6'4", about 175-180 pounds, so fairly slim, although believe or not a good 15 pounds heavier than I was. My options are somewhat limited by my height, but it does look like most companies make a tall bag.
I use a thermarest neoair as my pad. Haven't tried it in cold weather but I hear they provide quality insulation.

The Highlite looks pretty awesome, but I'm wondering if the Ultralite would be a better balance–lower temp rating, and a full zip for summers when it's too warm. My father has an ultralite and he loves it–he's got a pretty similar build to me.
I think I'll probably just have to go to the store and try out a few bags, see what feels good or not. I don't mind spending Western Mountaineering money, but I was hoping to spend less so I could get a bag that matched my GF's for zipping up purposes–she doesn't have one yet, but there's no way in hell I'm going to be able to convince her she needs a 400 dollar bag!

I use a WM Alpinlite 20*F bag, which has the same rating as the Ultralite.
It can be used in warmer temps by unzipping or opening it up as you suggest.

Above 50*F, I move to using my liner bag, which is the Montbell Down Thermal Sheet.
Below 20*F, I use the Montbell inside the Alpinlite as a liner.
Those are the temp points that I use with my 2-bag strategy.
It allows very light carry weight above 50*F, and light carry is nice when it's hot out.
At cooler temps and cold temps such as spring/fall weather, I use the Alpinlite alone, which is 2 pounds and might be a bit heavier than some other bags for those temps, but it can go down to 20*F, so it has a good range for normal use.
If it's going to be very cold and I need all I can get, then I bring both, and I don't mind carrying the 2.8 pounds combined total for that kind of situation.

I don't know what to recommend you for a liner bag, since the Montbell Thermal Sheet is too short for your use.
Perhaps someone can jump in with a suggestion.

Where do you hike? What is the coldest temp you expect? You might want to add insulation to the NeoAir below freezing, I use the NeoAir or Kookabay pads plus thin CCF. I was up high in Yosemite last late August and got a bit cold one night in my 30 degree Montbell. I carry layers to lower the comfort range of my bag, wool and down inner jacket work well for evening and early morning as well as upping the warmth of my bag.

I split my time between northern California and Minnesota, but probably moving back to MN. I'm buying mostly with the Boundary Waters in mind, but I do have another yearish in California. It can get pretty chilly in the boundary waters, even in the summer, and of course water is a concern with down, but a dry bag should work find.