You must have done AB testing to prove that expensive amplifiers sound different.

I am not saying that seperate amplifiers don't offer benefits, because they most certainly do. They increase dynamics, they allow for lower impedance loads in most cases, and they sound better when pushed because they have the sum of parts needed to power speakers at higher levels without clipping.
Preamplifiers make a small dent in the SQ, but speakers take the cake.
I was once a firm believer in snake oil, amplifiers that sounded different, and other nonsense. Now I believe that an amplifier that is well designed will deliver the same quality of sound while operating within it's comfortable power ratings.