I have considered B&W speakers fairly hard to drive due to the amount of phase shifting that occurs around the impedance of 3 to 4 ohms. This occurs at lower frequencies (higher amplitudes) which I understand makes the matter worse. This effect reduces power delivered to the speaker and increases the transistor power dissipation.
Stereophile did a review of these speakers, with their usual assortment of measurements and graphs. Here is their impedance curve:
The reviewer said this about power:
My estimate of the 804 Diamond's voltage sensitivity was 89.3dB(B)/2.83V/m. While this is slightly lower than the specified 90dB, it is still usefully higher than average. The plot of the 804 Diamond's impedance magnitude and phase against frequency is shown in fig.1. The speaker is a moderately difficult load for the partnering amplifier to drive. Not only does its impedance reach a minimum value of 3 ohms at 108Hz, there is a combination of 4.5 ohms and a 53° capacitive phase angle at 72Hz, and the impedance remains below 4 ohms for much of the midrange and the top octave.
For a "moderately difficult load" I think a 200 wpc amp would be sufficient, as long as the amp is stable below 4 ohms.
Remember that doubling the power to 400 wpc will produce a barely noticeable difference. To make a truly audible difference, a 10-fold increase in power would be needed. How many 2,000 wpc amps are there, and at what price? Even if you lower the bar to the square root of a 10-fold increase, about 3-fold, you have to ask how many affordable 600 wpc amps are there?
For the available info on the 804D, 200 wpc is fine. (Unless you're Walter
.)