0.5% THD is negligible and inaudible.
Don't worry about it.
I agree with the advice provided by myrison with one exception. A separate amp will make the peaks smoother at higher volumes as the headroom for separate amps over the all in one receivers is typically much larger.
As for the 'fatigued' sound, i would hazard an educated guess that it is a beer factor (or for that matter a factor of 'new reference'). When listening to a volume for awhile, it may not "sound" as loud as it did when you first turned it on. Adding the beer in there certainly alters the mental perception of it.
There is a friend of mind who actually starts talking louder the more he drinks. He doesn't even realize he's doing it but his wife uses it as a gauge as to when he's done and ready to go.
