In general bandwidth is defined by FWHM (Full Width at Half maximum) at a given amplitude (gain) or SPL level, which is 3db down for power as in SPL and 6db down for voltage/current. This is regardless of any amplitude variation within pass band.
As for what the published speaker specs mean, who knows? When I see a frequency spec with +/-Xdb, I take that as the max/min amplitude variation from the rated sensitivity across the stated bandwidth. That doesn't mean you're going to get 10exp(-X/10) less the power from the rated sensitivity at the two frequency extremes.
As for what the published speaker specs mean, who knows? When I see a frequency spec with +/-Xdb, I take that as the max/min amplitude variation from the rated sensitivity across the stated bandwidth. That doesn't mean you're going to get 10exp(-X/10) less the power from the rated sensitivity at the two frequency extremes.