Interesting read. I'm not surprised that differences were hard to discern between high quality amps, but I'm surprised the Pioneer receiver did so well. I'm definitely in the category that there are differences between amps, but perhaps it does simply come down to available clean (and quiet) power (which is a product of the internals, of course). At low enough volumes, it makes sense that it would do okay, though I'd expect a higher noise floor with it that would be detectable. Perhaps not. Crank it up and things would change dramatically.
I guess my main sticking point is the blanket assumption that a cheap receiver is going to sound just as good as decent quality separates. This is simply not the case in real world use. A cheap receiver's amplification is generally quite limited. Even the modest Outlaw 7075 is going to trounce a cheap receiver's internal amps. Most people aren't going to listen at low volumes all the time in a theater environment, and having more clean power on hand is always a good thing when you want it loud.
Perhaps this was the biggest difference in the systems I compared. The separate amplifier had more reserve power and handled transients better (the test was not at low volume, though not at reference levels, either). Whatever the reason, it did sound cleaner and clearer to me. The differences I heard were not due to my imagination.
At low volumes, I have no doubt the difference is minimal. My recommendation for separates (or a more expensive receiver) is based on real world use. Throw in lower ohm speakers all around, and this argument makes even more sense.
