It depends on your speakers, how loud you listen, and how far you listen.
I think most people don’t listen to “Reference THX Level” and most systems don’t need an external amp.
I'll repeat what PENG has repeatedly said: any amp can power even a 1-ohm speaker if the volume is low enough.
My first subwoofer was a 10" NHT subwoofer with an 80 Watt amp! Can you even find a subwoofer these days with 80 Watt?
And I was using this single 10" 80W subwoofer in an 18'x20'x10' open living room and shook all the walls like an earthquake.
I've used a 60WPC amp to power my RBH SX-1010N subwoofers (each sub has dual 10" Woofer) to as loud a volume as I am comfortable with, which means shaking all the walls and my bones like an earthquake. Yes, I absolutely love larger-than-life-bass. If "Reference THX level" is louder than this, then I don't want THX level because I will go deaf.
Like HD says, it depends on your use/goal/need/desire, etc.
So I am sticking to my belief - most AVRs in most systems don't need external amps. External amps are meant for separates (pre-pro and preamps).
But if you want to add a great amp to your AVR, that's cool too (even if I don't believe in it).