Joe, Ian or Alan would probably have to describe the exact conditions under which Axiom made the measurements, but in general the plus or minus 3 db would indicate that from whatever frequency happened to measure as a high point(say 800hz)that all the other frequencies from 37hz to 22Khz fell no more than 6 db below that( therefore making a +/- 3 db "window"). Almost always it would be the lowest frequency(37hz)where the response would be 6db down. The additional +3/-9 measurement would indicate that at 27hz response was down an additional 6db from what it was at 37hz, i.e. a total drop of 12db from the high point.
Bear in mind that these measurements were most likely made under anechoic conditions and that the room gain when listening in an average room would boost the bass response significantly.
