What is the differnce between a class "A" stero amplifier and a class
"AB"? Of what significance is linearity as power changes in face of
changing reistance? For example a Krell amplifier rated at 300 watts at
8 ohms resistance will measure 600 watts at 4 ohms resistance while a
Bryston amplifier produces 500 watts at 8 ohms resistance but only 800
watts at 4 ohms resistance.

The information you want may not be available on the internet, so I
will give it a shot.
 About 80 years ago amplifier designs were A,AB,B and C. B never was
popular and C is suitable only for RF = radio frequency ampliers. The
terms were dropped, generally when transistors grabbed most of the
market, however the output stage of transistor amplifiers is almost
always biased AB for ten watts or more.
 The answer about the impedace verses power makes no difference if
these are tube amplifiers, transistor amplifiers or some other solid
state device. It is a minor design difference, how much power you at
other than the design optimum impedance. Often the power is limited in
a variety of ways to prevent speaker damage, and/or amplifier damage
from excessive input. The output may be more limited at base
frequencies as speakers are more valnerable at base frequencies and
the speaker impedance is typically higher at treble frequencies making
damage of anything other than ear drums unlikely.   Neil

I do not claim to ba an audio expert, but as an audiophile and
enthusiast I can tell you what I know:

The main difference between a class A and class AB amplifier
topologies is the circuitry. The Class A has one voltage rail, and can
handle only one output device per channel. The Class AB has two
voltage rails, and can handle multiple devices per channel.

In theory, the power should double with a reduction by half of the
resistance, but there are basically one reason that dont make this
practicable to the given examples.

Most amplifier manufactures have some sort of power regulating
cicuitry built into the amps, to protect other internal components.
So, you dont allways get full power when you expect it.

Let's see, time to crack open the old textbook.  September '82, that
brings back memories.  Electronics Devices, Discrete and Integrated
Circuits (Seidman, 1977) states: "In class A operation, output current
flows for a full 360 degrees. In class B, the output current flows for
half a cycle, or 180 degrees. The case in which the output current
flows for more than 180 degrees but less than 360 degrees is a class
AB operation.  If the current flows for less than half a cycle, we
have a class C operation."

So, interpreting that for audio, the class A amplifier provides the
greatest fidelity since the output will be the same as the input.  The
class AB amplifier introduces some distortion since it does not
amplify the full 360 degrees.  However a class A amplifier is not very
efficient, where efficiency is defined as the ratio of the ac power
delivered to the load compared to the dc power supplied to the
collector circuit.  A series-fed class A amplifier has a maximum
efficiency of 25%, a transformer-coupled class A amplifier has a
maximum efficiency of 50%. "In practice, a transformer-coupled class A
amplifier is designed to have an efficiency of 35%-40%" (Seidman,
274).

Class B audio amplifiers use a push-pull configuration where each
transistor amplifies for 180 degrees of the input signal.  One
disadvantage is the distorion at the crossover, hence crossover
distortion.  To minimize crossover distortion, the transistors are
biased so that they are conducting slightly.  Such biasing provides
conduction for more than 180 degrees, resulting in class AB operation.
A class B push-pull configuration provides a maximum efficiency of
78.5%, a class AB push-pull configuration approximately the same.

The maximum output power in a push-pull circuit is determined by the
(square of the voltage) divided by (load impedence X 2). The actual
power delivered could depend on a number of factors such as power
limiting protection circuitry, amplifier design and voltage/current
limitations.