Added an Emotiva BasX A3 amp (140w/ch) this week and it opened up the front stage- dynamics & clarity, especially dialogue.
I found this to be the case with my Integra DRX3.4 - I believe it to be caused by the AVR needing to drive speakers with low impedance dips (in my case down to 1.6 ohm) and not being able to provide the current to meet those needs, or in many cases, instability in the negative feedback loops, which can be triggered by low impedance speakers...
Either way, "difficult" speakers are a torture test for amps, and can frequently make different amps, sound different.
The same amps, driving "easy" 8 ohm speakers, all sound identical.
So yes, there is some science behind it, and it all comes down to matching speakers to amps (and vice versa)... amps that can handle difficult speakers, can handle easy ones too... but the opposite is NOT the case.
Most AVR's on the market (with a few high end megabuck exceptions) are current constrained, and have issues driving difficult speakers.
High current AVR's (like previous generation Onkyo SR876, Integra DTR70.4 - which I have owned) handle it fine... current generation Denon AV1 should handle difficult speakers just fine.
One thing all the "heavy duty" AVR's have in common (other than being more expensive!) - is their weight, the transformers and capacitors in the high current power supply typically double the weight of the AVR compared to its smaller mass market brethren - the high current ones are typically circa 50lb / 25kg, where the mass market models are around 20lb / 10kg.