That's somewhat true, especially since the FTC many years ago declined to enforce any kind of specifications. So AVRs specify power into 2 channels driven, 8 ohms only.* From years of reading tests, they ALL droop their per-channel power as more and more channels are driven. Basically the power supplies are just not super strong. Continuing what someone pointed out earlier, from 100 watts to 150 is just like1 step of the volume control more, because our hearing is logarithmic. I consider all AVRs to have essentially the same power, and if you want more REAL power then you need something rated at least 300 watts continuous into FOUR ohms.*
Adding an external amp (a POWER amp, not that tube thing you linked) for say the front L/R means the AVRs power supply is not as strained and can deliver closer to rated power in the remaining channels since it won't droop as much. This is also true of adding a subwoofer, now the AVR is not strained by the low bass frequencies, which are the hardest for amplifiers. This is often NOT true for 2-channel equipment, which stupidly usually have no highpass filtering, even when they have a “subwoofer” output!
Here are "Head_Unit’s Rules Of Protection":
1) If when things start to sound distorted or odd you TURN IT DOWN, you are unlikely to ever break anything.
2) If you constantly "turn it up to 11" you will break something.
NOTE: size and power ratings of speakers and amp do not affect rules 1 and 2; there is NO need for "matching." Speaker specifications are 92% meaningless (and I say that as a loudspeaker engineer). Specs for amps are not thorough since they are measured into resistors for pragmatic reasons and speakers are not resistors at all.
*Amps' 4 ohm or even 2 ohm rating is the most meaningful even if your speakers are 8 ohms. Should be 20-20k Hz, distortion under 1% or it's baloney.