1. The “more is better” trap. With audio amplifiers, more power is, within reason, better. More power means more headroom before you reach the limits of what your power amplifier can deliver. With movies, music, and games, there is a tendency for highly dynamic signals (a crest factor of 6:1 is typically cited). When a big explosion happens in a movie or a kick drum in music, there are some advantages to having extra power you don’t need. If the amplifier is clipping, it’s not going to sound good. More power means more SPL before you reach the limits of your amplifier and it clips. Lower power amplifiers can deliver excellent sound, but you do loose some headroom. Also keep in mind that a 10X increase in power results in a doubling of SPL (Volume), so small differences in power aren’t as important.
2. Power specs are largely unrelated to the performance of an amplifier in an actual home theater environment. Running continuous sin waves at 1kHz for power for testing does not present the same kind of dynamic signal that actual content does. Sin wavs are much more stressful to amplifiers. That being said, you need a scientific way to evauluate what a power amplifer is capable of. Also, measuring a single channel for a spec or measuring at 10% THD is not necessarily bad. The point of power ratings is to be able to compare two amplifiers. If both amplifiers share similar topologies and are measured the same way, you can compare the relative power output of the two. If you are comparing an amplifier that shares a power supply verses one that has individual power supplies for each channel, then comparisons will break down. You are correct that everyone should be measuring all channels at once, eliminating the necessity of knowing anything about the amplifier design to make a comparison. The only body in the US that could enforce this is the FTC, and they have decided they have better things to do. Point being, power rating are not related so much to actual use of the amplifier in practice, but can be used to compare output capabilities of two amplifiers.
3. If you have cost constraints, buy the best speakers first and get a cheap interim amp that you can upgrade later. You get far more in performance for your money from speakers than from amplifiers. If you have to make a choice, get the expensive speakers and the cheap amp. There are very few amps these days (excluding tubes) that have distortion much over 0.2%. Speaker distortion hovers closer to 3%, even on the best speakers. If you get an amp with decent distortion across frequency and level, low noise, enough power and low crosstalk, you’re amp won’t be the weakest link in the chain.

NEVER buy speakers based on the specs alone. There are two many specs you need to know, many of which manufactures do not share with consumers for marketing reasons. It is preferable to try speakers out in your acual listening envoironmment as well before making a final decision. This is true to a lesser degree on amplifiers as modern amplfiiers are much more transparent than other components in the signal chain. You can probably get away without doing the careful listening eauluations required for selecting speakers when selecting an amplifer.

The amplifier portion of the audio industry is frought with false claims, specmanship, and marketing over performance. I pity the consumers who do not have in depth knowledge of amplifers who have to wade through the market.

1. The “more is better” trap. With audio amplifiers, more power is, within reason, better. More power means more headroom before you reach the limits of what your power amplifier can deliver. With movies, music, and games, there is a tendency for highly dynamic signals (a crest factor of 6:1 is typically cited). When a big explosion happens in a movie or a kick drum in music, there are some advantages to having extra power you don’t need. If the amplifier is clipping, it’s not going to sound good. More power means more SPL before you reach the limits of your amplifier and it clips. Lower power amplifiers can deliver excellent sound, but you do loose some headroom. Also keep in mind that a 10X increase in power results in a doubling of SPL (Volume), so small differences in power aren’t as important.
2. Power specs are largely unrelated to the performance of an amplifier in an actual home theater environment. Running continuous sin waves at 1kHz for power for testing does not present the same kind of dynamic signal that actual content does. Sin wavs are much more stressful to amplifiers. That being said, you need a scientific way to evauluate what a power amplifer is capable of. Also, measuring a single channel for a spec or measuring at 10% THD is not necessarily bad. The point of power ratings is to be able to compare two amplifiers. If both amplifiers share similar topologies and are measured the same way, you can compare the relative power output of the two. If you are comparing an amplifier that shares a power supply verses one that has individual power supplies for each channel, then comparisons will break down. You are correct that everyone should be measuring all channels at once, eliminating the necessity of knowing anything about the amplifier design to make a comparison. The only body in the US that could enforce this is the FTC, and they have decided they have better things to do. Point being, power rating are not related so much to actual use of the amplifier in practice, but can be used to compare output capabilities of two amplifiers.
3. If you have cost constraints, buy the best speakers first and get a cheap interim amp that you can upgrade later. You get far more in performance for your money from speakers than from amplifiers. If you have to make a choice, get the expensive speakers and the cheap amp. There are very few amps these days (excluding tubes) that have distortion much over 0.2%. Speaker distortion hovers closer to 3%, even on the best speakers. If you get an amp with decent distortion across frequency and level, low noise, enough power and low crosstalk, you’re amp won’t be the weakest link in the chain.

NEVER buy speakers based on the specs alone. There are two many specs you need to know, many of which manufactures do not share with consumers for marketing reasons. It is preferable to try speakers out in your acual listening envoironmment as well before making a final decision. This is true to a lesser degree on amplifiers as modern amplfiiers are much more transparent than other components in the signal chain. You can probably get away without doing the careful listening eauluations required for selecting speakers when selecting an amplifer.

The amplifier portion of the audio industry is frought with false claims, specmanship, and marketing over performance. I pity the consumers who do not have in depth knowledge of amplifers who have to wade through the market.