Almost any competently designed solid-state amp will have pretty much a ruler-flat frequency response from at least 20Hz to 20kHz - on a test bench, driving a resistive load.
The real world is different and a much more scary place for an amplifier—the impedance of actual loudspeakers changes with changing frequencies, sometimes with astonishing speed and degree. The Alexx V and XVX are particular cases in point, each with very low minimum impedance points.
This means that not only will an amplifier’s output capability vary with frequency, but once you start driving it with a musical signal, its capability in responding to the dynamics of music comes very much into play. That’s a critical part of this, as no two amplifiers will be the same in that regard.
So, in the real world and real systems, two different amplifiers will drive the same speaker very differently, with a different dynamic and bass response.
I think the whole point of Roy Gregory’s post was that while it would be nice if changing an amplifier were a simple thing, it’s just not. In reality, you are changing both the relationship between the amplifier and speaker but also the relationship between the speaker and the room. If a speaker is being driven differently by whatever amplifier, then the speaker will drive differently what it is driving. And that is the room. It’s been my experience that it’s essential to consider speaker placement when you change the amp(s) - or pretty much anything else in the system! That’s just physics...