What's the difference between a "Box" Titanium and a regular Titanium? Or is that just a misprint?
I am not sure, but sometimes "box" refers to larger room in the bag for your feet.
If you are interested, call REI customer service, they are excellent.
fWIW, I think you could usually add about 15 to 20 degrees to what they advertise the bags for temperature rating. There is no real standard. The original measure relates to being able to survive in a sleeping bag. Most want to be comfortable not simply survive.
Mountain Hardwear is a good brand.