Me, too, am very biased, so at one point I have a dozen of power amps for just three 2ch stereo systems, now down to just 3 that has much more juice than I need, plus 3 Fosi, for the same stereo systems. In reality I would have been fine if I just keep the 3 little Fosi amps.One of those posts where can't quite quote science or math, so probably worthless. And also points to inconsistent experiences, which does not help either.
Nonetheless, I don't feel like my Denon 6700H was ever able to drive very efficient Klipsch 7.x.4 set beyond -10/-15 dB master volume beyond the point where the experience started to sound harsh. After adding some external amps to the setup listening to reference volume even in the 2500sft room became quite possible and really enjoyable. Obviously you want to limit such volume to 2 or less hours per day, or not even every day, so my use for it was the new or older movies I thought would be worth of it.
Interestingly enough, the acoustics of the larger loft where I have another HT, is rarely calling for volumes beyond -10dB, despite the fact that distortion in that system should overall be lower than in the smaller system with Denon/Klipsch and amps are overall more powerful and AVP better, albeit speakers more demanding. Never tried that system with AVR, so can't really tell if would be happy for AVR to drive it.
While difficult to find a common denominator between the above, it might be that you should try and experiment, with an open eye. I am unfortunately naturally biased to amps, but that is my problem. The better way to experiment with external amps is to consider them as additional strain on your budget and that they would need to prove themselves big time before you keep them. If you just think they will be better without the chance to critically evaluate them, chances are they will sound better as well. So "negative" bias to amps, will actually be a positive one in evaluating them.