I always consider a compression sack a heavy way to overcompress and ruin a piece of insulating gear; and it's usually a piece of insulating gear that was the cheapest, but not the best choice. I just put my sleeping bag loose in a trash compactor bag, and let it fill up the rest of my pack volume. And most manufacturers of quality down gear will tell you not to overcompress it. I know WM says not to use a compression sack, or stuff sack smaller than the one they send.