...
But if we are dealing with two well-made, properly functioning amplifiers, and if we are using them within their capabilities, and if we carefully level match them, then humans cannot hear any difference. This is a simple fact: NO ONE HAS EVER DEMONSTRATED THE ABILITY TO HEAR ANY DIFFERENCE IN SUCH CIRCUMSTANCES. Despite years of trying. Mostly, though, people do not properly set up a test, and imagine that they can hear a difference (among those who claim such an ability). And, of course, if the amplifiers are not level matched, they might hear a difference, or if one is malfunctioning, or being used outside of its capabilities, etc.
So, the upshot is, an amplifier can make a difference, but typically it doesn't.
...