I ran the Denon for 6 months and was familiar with how it worked... yes it always got warm.
I ran it in Eco: auto, all the time.
It definitely changed temperatures when I connected more speakers, and then ran even hotter when I added the tower speakers. It became so hot I was worried about it - it nearly burned my hand. That never used to happen driving 2 bookshelf speakers.
(Granted, that was with 4 Polk LSiM tower speakers + their huge center channel, listening @ -10 to -15db. The Denon was able to do it, but it became intensely hot. )
Not being an audio engineer, common sense would me that's not a good sign for the health or longevity of that receiver. In pre-amp mode, the Denon hardly even gets warm now.
As to the 2nd point: Yes, the sound quality did improve. It's hard to quantify exactly.
The Emotiva definitely added more bass impact and slam, that was the most noticeable thing right away.
The sound is more clear at higher volumes and was a little more "real" sounding at all volume levels. But even on normal volumes at the same volume as before, the sound seems more realistic. It is not a night/day difference in clarity, no... but I was very familiar with how the Denon sounded having listened to it exclusively for 6 months.
When I added the Emotiva in the same system, the sound became more clear and slightly more expansive, just from not using the Denon's amps.