If I look at the PK Metric of original vs amplifier recordings for a given track, they look similar no matter which amplifier. So it seems difficult to identify a better amplifier this way. Maybe that is just what we are up against when the differences in direct amp to amp comparison are small.
Maybe that's because the differences are barely audible, if at all? You can always find a measurable difference between two analog devices if you look deep enough. The question is, at what point do you stop looking and decide that they are pretty much equivalent?