Most of the time, you will never need an external amplifier.
Although I have plenty of external amps, I have never really needed one.
Requirement for external amps depend on 3 things:
1) room size
2) speaker sensitivity/resistance
3) volume
My room size is not very large. My HT room is 14' x 17' x 15' ceiling. My Family room is 18' x 20' x 10' ceiling.
I can tell you that my brother now has my old NHT SuperZero + SW2P sub. He also has my old Harman Kardon (HK) 50 watts per ch (WPC) AVR. He does not use an external amp at all. His family room is 20' x 20' x 12' ceiling. He can play movies in 5.1 at reference volume 90dBA from his seat just fine. No clipping whatsoever and the 50WPC HK AVR never goes into circuit protection mode.
So a Denon AVR (or comparable) with 100WPC or more should be just fine.
Unless your room is very large and you play louder than 90dBA.
But it's always nice to have ext amp outputs, even if you never need it.
In other words, if you have the chance to have ext amp outputs for the same price/feature/reliability, etc., why not?
