I too have JBL speakers with the same frequency response. I've never heard of 
the speakers response to go lower with a higher volume. You may be hearing 
differences with room acoustics, closeness of the back port to the wall, and 
general speaker placement. Speaker placement has A LOT to do with frequency 
response. The manufacturers response was done with close micing of the 
speakers cone, and everybody knows that the room and placement can add of 
subtract how much 'stuff' you hear. Stuff being enhanced bass or extended 
highs. It's almost like the 80Hz 'cabin gain' found in automobiles.....
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