I think people do understand your question "Or is it money spent for no significant overall improvement? " that "improvement" is not about "loudness". You don't have to believe anyone, or "articles" if they/those gave their subjective views, or opinions. Just look at the facts that the Emotiva amp you mentioned did not show any evidence it would improve based on available measurements such as those on Stereophile, other that it obviously is more powerful than your AVR. That's why some of us are telling you you won't likely (I always prefer to insert "likely") hear a difference if your AVR can cover your current power need, with reserve. Other than that, no, "the quality would not be improved, at least not to the point it would result in a audible difference. Case in point, the X4400H has not been measured, but the very similar X3600, 3700, 4700 have been measured, they all have virtually identical power amp section, especially the X4700H.
Emotiva XPA Gen3 two-channel power amplifier Measurements | Stereophile.com
If you have questions on those measurements, we can likely help.
Now if you want to be sure, it definitely won't hurt to go for the amp anyway.