i caught this in a description of b&w speakers,just somebody elses opinion i guess


Sensitivity combined with impedance indicates how much amplifier power is required for the loudspeaker to play at satisfying volume levels. When comparing this spec to low impedance (4 ohm) designs, you must add 3 dB to the figure, since 4 ohm loads draw twice the current from the amplifier, providing the illusion of greater sensitivity, while placing more stress on the amplifier?s power supply. The moderate sensitivity of the Pearl indicates that small amplifiers can achieve realistic levels in all but the largest rooms.

System impedance indicates whether the speaker presents a "hard" or "easy" load on the amplifier. True 8ohm speakers are less amplifier and cable sensitive, and partner well with a broader range of amplifiers, including tube designs.