It is likely, I am sad to report, that the difference in sound was because the amp happens to have a higher input sensitivity, so that it is just a bit louder than the AVR at any given volume control setting. IOW, it is likely that just turning up the AVR a few dB will yield the same apparent sonic improvements.
It is really hard to accurately compare two different amplification devices, what with needing precise level matching and, for humans, a method to take away our ability to know which device we are listening to at any given moment (or on any given channel).
Admittedly, at my listening levels with my speakers, I likely hit my max level (around 95 dB, with dialog at around 75 dB with less than ten watts). Unless one of the amps is designed to be inaccurate, I've come, slowly, to the realization that transparent is transparent, inaudible distortion is inaudible, and Ohm's law prevents, in an absolute kinda way, 10 watts from a 50 watt amp from being any different in any respect from 10 watts from a 100 watt amp (assuming both are designed to be accurate - - there are some, especially AIUI British amps from the early CD era, that are intentionally designed to be inaccurate - - that is, to have a little higher-frequency rolloff. Those may well sound different in careful auditioning, since they ARE different in audibly meaningful ways).