The way it's been explained to me, and makes sense, is that amplification is a very mature technology. The big players all have the same goal of transparency and low distortion output, and have that part of it figured out with most having such ridiculously low distortion numbers the differences are inaudible. Most amps when operated within spec with all else being equal should sound the same. There are some exceptions like tube amps or difficult to drive speakers, but for the most part, in my experience that's been the case.
Here's an interesting article that I like to link of a carefully set up dbt between some high dollar "audiophile grade" gear with fancy cables and an entry level setup using the same speakers with interesting results.
There aren't a lot of examples out there because proper DBTs are a pain to set up and get right, but I'm unaware of any that have had better than chance odds of listeners able to tell one from another.
