I also think bag ratings are based on having wool base layers not cotton.
+1 for Mountain Hardwear. I have a Tallac 20º that belonged to my uncle and it still goes strong. I even ripped a baffle washing it and sent it to them and they repaired and return shipped it no charge (had to pay $7 to ship to them).
Just slept in 24º weather with the my bag a swag tent and merino base layers and I slept great