Let's look at Amir's
Benchmark AHB2 review, specifically at this graph. This is how to read it.
Horizontal scale: power output in Watts at 8 Ohm. Easy enough to understand, the more we go to the right, the more power the amplifier makes.
Vertical scale: THD+N Ratio. This one needs more explanation. It is the ratio of THD (Total Harmonic Distortion) and Noise to the output signal. It can be expressed in percentage or dB. You can switch between both by using a
THD Calculator. Note there are two expressions, THD and Noise.
In a typical amplifier, noise remains constant. This is why the THD+N ratio appears to decrease as amplifier output goes up. It's not the noise that is going down, but the power that is going up.
At some point, the amplifier can't take it any more ... it can't supply enough current, and it starts to clip. When it does this, THD shoots through the roof. You can see it happening here at about 100W. In fact you can see the THD+N climbing very slowly from about 15W to 100W, at which point it takes off like a rocket.
You can gain more information if you probe the amp with different frequencies. Most amplifiers, including this one, can't deliver the same maximum power output before the amp starts to clip. Here you can see the distortion rising at different rates from 2W up, and some frequencies clipping earlier than others. What you want to see is all the frequencies bunched up together.
Maybe Amir has tested a hilariously bad amplifier. I don't know, I don't usually read amplifier reviews, I don't find them particularly interesting.