The 802 impedance is all over the place and needs an amp that can handle the swings even though they are 90db efficient. Lots of speakers are like this though. Basically they need lots of current and with lots of current comes lots of power. It goes back to Ohm's Law (I=V/R) as the resistance drops the current goes up (voltage is constant in the home). Of course this is a very simple explanation and the capacitors in the crossovers will complicate things. There are lots of designers on audiogon that could explain it better... electrical was never my strong suit in school.
"I estimated the B&W 802D's voltage sensitivity as 89dB(B)/2.83V/m, within experimental error of the specified 90dB figure and usefully higher than the average of the more than 500 speakers I have measured in the past 16 years. The B&W's impedance plot (fig.1) reveals the speaker to be moderately difficult to drive, with a magnitude that drops to 3 ohms throughout the upper bass and an awkward combination of 4 ohms and 50° electrical phase angle at 60Hz. In addition, the very high peak between 2 and 3kHz, resulting from the tweeter/midrange crossover, will give a somewhat forward tonal balance with amplifiers having a significant source impedance; say, tube models."
look at the first graph.
http://www.stereophile.com/content/bw-802d-loudspeaker-measurements
"I estimated the B&W 802D's voltage sensitivity as 89dB(B)/2.83V/m, within experimental error of the specified 90dB figure and usefully higher than the average of the more than 500 speakers I have measured in the past 16 years. The B&W's impedance plot (fig.1) reveals the speaker to be moderately difficult to drive, with a magnitude that drops to 3 ohms throughout the upper bass and an awkward combination of 4 ohms and 50° electrical phase angle at 60Hz. In addition, the very high peak between 2 and 3kHz, resulting from the tweeter/midrange crossover, will give a somewhat forward tonal balance with amplifiers having a significant source impedance; say, tube models."
look at the first graph.
http://www.stereophile.com/content/bw-802d-loudspeaker-measurements