To make this a bit simpler, Class A amps will typically generate the most heat because they draw full power all of the time. Class AB amps will generate less heat but the amount will vary depending on whether they are heavily biased into Class A (more heat) or less. Class D amps generate the least amount of heat. The situation where speaker efficiency will make a significant difference is if you go with very high efficiency speakers, because you can then use a very low power tube amp.
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