You appear to have done everything correct in REW and in your measurements. I don't really know what to think. We haven't had this situation before.
Does the very low end appear to be weak in reality, and it's only the graphed response that shows it poor?
Have you tried your sub in a small room as a test to see if it then produces good extension - (bedroom)?
Have you considered perhaps there is a problem with your meter. Could you borrow another one to test that out?
brucek