So why does Axiom and others call their amps a digital design?

Good question. I don't know the history of that one. Maybe it has to do with the fact that the FETs turn on and off really fast so for slang they call it "digital".

The A1400's efficiency is proportional to the power sourced from it. So for me playing at a half watt, it's probably as efficient as a lightbulb .

I didn't think light bulbs were all that efficient. They shed a lot of heat.

Sorry, I have no real point here. When I'm overtired my brain woks in odd ways.

That's right Murph. I don't know how efficient a lighbulb is exactly but it's probably less than 10%.

Not sure if the amp utilizes this or not but I think it explains where the digital in Pule width modulation comes from

http://www.embedded.com/story/OEG20010821S0096

also found this really technical guide to the Basics of class D amplifiers

http://www.irf.com/technical-info/appnotes/an-1071.pdf

The International Rectifier discussion is excellent, but the embedded link doesn't use the term "digital" in the generally accepted manner, and some might say is simply incorrect. Pulse Width Modulation of a continuous analog waveform doesn't involve any digital sampling, and although the max/min appearance of the PWM waveform has been said to be "digital-like" in appearance, it's still just a modulated continuous analog waveform. The analog waveform "concealed" within the PWM waveform is revealed when the low-pass filter removes the PWM frequency and its harmonics, leaving the original analog waveform.

Yeah, that's right. I think some marketeer called this "digital" to be "hip" with the times. That's the difference between architecture and marchitecture .