wattage

Almarg, would you please elaborate on why SPL does not decrease at 6 db per doubling of distance on planar speakers? I thought that figure was a constant for propagation in Earth's atmosphere rather than being related to the type of device producing the sound?

77jovian, the main reason that SPL decreases as distance increases, in a reasonable home listening environment at least, is not related to atmospheric attenuation. It is the result of the sonic energy "spreading out" over a progressively larger cross-sectional area as distance increases, resulting in a given cross-sectional area receiving a progressively smaller fraction of the total energy.

In contrast to a typical box speaker, though, where a given frequency is likely to be emitted by one or two or three relatively small drivers, a large panel speaker will tend to "beam" most frequencies, resulting in a lesser degree of "spreading" as distance increases. Or putting it another way, as the listening distance increases the listener will tend to be "on axis" with a greater amount of the panel's area. And also more equidistant to the various sections of the panel's area, resulting in less difference in the arrival times of the sound emitted by the various sections, resulting in more coherent summing of the different arrivals.

Regards,
-- Al

As I read your post Tennisdoc40, I was thinking about the SPL drop-off with each doubling of distance from the plane of the speaker...
I scrolled down & saw that Almarg had already addressed this point. :-)
I'm thinking exactly along Almarg's lines.....
To make the music flow effortlessly - meaning playback of all the peaks without feeling the music is contained - you will need a lot of watts. 1000X the spec'd wattage at 8 Ohms, just like Almarg wrote.

Ratio of peak to average power required, as Al said, is very
high. Amplifier's power specification alone is only useful
for listening to continuous sine-waves. Otherwise 10W
amplifier with big headroom can sound much louder than 100W
amplifier with small headroom. For instance, in class A amps
headroom is very limited. Higher output bias current would
make amplifier very big, heavy and expensive. Class AB amps
don't have this limitation - headroom can be higher.

In addition power specification are often very vague.
Continuous power of the B&O module (200ASC) in my class D
amp is 40W. FTC Power is 55W. Rowland rates it 200W while
Bel Canto rates it 300W. What is it?

This 1000x wattage issue is misleading. The math works and so does the logic, but it's not how your ears and brain hear it. Clipping and the dynamic compression that it produces can be very hearable and annoying, or it can be hearable and somewhat pleasant sounding, or it can completely unhearable. It depends on a number of factors.

Even with the newer class D amps, high quality-high wattage amplifiers are still expensive. In allocating money in a system I'm not sure it's wise to spend based upon what happens 1% of the time. I'm repeating myself, but a well designed 50-100 watt amplifier should suffice for the vast majority of circumstances.