Okay so this is for the benefit of those of you who like me are pondering whether to buy a good external amp like NAD M28 to use instead of built-in amps in your Denon AVR (which are objectively good buy not as good as NAD’s).
I bought this NAD and had it for ~ 10 days or something. Initial impression has not changed: no perceptible difference in sound quality except at my peak listening levels in movies ( volume ~ 74 on Denon x6500) when NAD somehow is not harsh at all but Denon just a bit is. Why it is the case I don’t know, obviously Denon’s amps are not clipping at this volume. I had the same experience - zero perceived change in sound quality - upgrading to x6500 from x4500, while the upgrade from x3400 to x4500 brought a very substantial reduction in audible noise (x3400 must have been pretty lame).