I'm not that familiar with Sunfire, but I quick review of the owners manual for that amp reveals a 0.5% THD. That's 10x the THD of the Denon you have now. A 70W gain from 130W is only about a 1.5db gain. You will not notice that headroom.
I would recommend going for a quality 2 channel power amp rated at least 400W per channel to get a noticeable headroom improvement. Try to get one that is in the 0.07% THD range or better.
I do a lot of multi channel SACD etc. How would adding just 2 channels of 400W affect that?
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.
