Dynamic Headroom

I disagree with the weak power supply description also and that "headroom" is more important for tracking the musical signal properly. A good example is an amplifier I happen to own and I own it for the design we are discussing. The amp is rated at 60 watts per channel, but this is one of the most power "sounding" amps I have ever heard. A glance inside reveals a huge transformer with capacitors the size of the oil filter on your car. With something like 8 db of headroom, this amplifier seems impossible to run out of power. It played Vandersteens louder than any amp regardless of power rating including some well known 200 watt per channel amps. My neighbor said he liked feeling the bass on his face. Not that I make a habit of playing music this loud. It was a demonstration of what this system was capable of. This type of amplifier is also a good choice for speakers with a lower power rating. They will play louder without bottoming out on large scale dynamic peaks.

Atmasphere, Your statement is simply not true. The class of operation has nothing to do with headroom. Headroom is the amplifiers ability to perform beyond it's rated power for a short amount of time, like during a dynamic peak in music. Class A operation eliminates switching distortion found in class AB amplifiers. Alternatives are A AB or AB1 which are high biased AB and run in Class A to a higher output rating than AB.
Also, your atatement of "headroom specs are a way to make cheaper amps look better" is also false. A much larger power supply is required to add headroom to an amplifier and the power supply is where the money is. The larger the storage capacitors the larger the tranformer has to be to keep the capacitors charged. This is where headroom comes from.
All of Krell's pure class A amplifiers have 3db of headroom. Classe Audio class A DR3B and DR3VHC have 8db of headroom. Bedini class A amplifiers have 3db of headroom. Just to mention a few.

Different philosophies by different designers I guess. One designer claims to have something and the other designer claims it doesn't exist.