Each speaker in the Nautilus line-up comes with a individually measured frequency response trace from 20Hz to 25kHz. The specs that B&W uses in their manuals and web site are for Frequency Range (-6dB low/high roll-off points), and Frequency Response (the ±3dB low/high threshold frequencies on the reference axis) which should also be interpreted as roll-off points for practical use.

While the traces are certainly not ruler flat between these extremes, the sound pressure variation is significantly less than 3dB within the speakers usable range. It should be emphasized that there are more significant characteristics of a speaker's voice that contribute to good or bad sound reproduction. These characteristics cannot be EQ'd out of a system, and deal with the accuracy, phase, and the degree to which the speaker can eliminate (never completely) unwanted sound (noise) generated by the drivers and enclosures. Off-axis response is also an important quality that is often overlooked by simple specifications. Not everyone can be at the sweet spot.

Respectfully,
Allan