Tested using REW built-in CEA bursts, 50 Hz to 12 Hz. This is not a scientific test, since it wasn't done properly outside, and with Samson USB mic, which has pretty flat response, but doesn't have a cal file.
The graph seems too good to be true to me, seems like RMS average goes even beyond 10 Hz, is this due to my room specifics? My room is 300 sq ft, with cathedral ceilings.