"Konrad, what specification would be more meaningful than loft?"
Down fill amount and fill power. Too many variables go into loft, including baffle height, baffle width etc. I'm pretty sure there is a BPL article floating around here stating that loft isn't a reliable measure.
Also, from Richard N.,creator of the infamous new paradigm of warmth chart, "Trying to determine the warmth of a garment by just measuring its loft is a measure of futility."
"Loft is one of the few specifications that an ordinary consumer can check while in a sleeping bag showroom."
I would hope product reps would be able to give you fill weights off hand, otherwise they aren't very good at their jobs
"If down is distributed suboptimally in a sleeping bag, you get cold spots."
If you don't have enough down to begin with, you are just left cold.
"Wait a second. If the bag was ordered yesterday, then wondering about it today doesn't really accomplish much."
You can always return stuff, even used in the case of EB