Understanding Power from amps

[Igniter]

Hi All

Looking for an easy explanation on what the following type of classes are:
Class A
Class A/B
Class D
Valve

And why watt rating on valve is so low when compared to non valve... e.g. valve 20w = class A 100w

Plus the best way to understand how to best match an amp to speakers ? whether valve or non.

Much appreciated.

 

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[Jacobus]

 

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[DACMan1]

 

[KenMasters]

 

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[Igniter]