This is probably both an overly-simplistic question as well as a paradoxically controversial / complex one, but how is one to be reasonably confident that they wouldn't benefit from an external amp by ear alone - the obvious answer of "try one and see!" aside?
From a pure sound quality perspective, if using an AVR at your maximum listening levels results in:
No audible distortion
No audible clipping
No overabundance of heat / shutdowns from the AVR
Can you be fairly assured that you're not "leaving performance on the table"? Or are there other factors that might weigh into it? Speaker reviews often state things like "these are tough to drive, so make sure you have a dedicated amp with plenty of power on tap!". If you're hitting all of the above points with an AVR with no issues, is that really the case anymore?