The Richard Clark Amp Challenge is a listening test which goal is to prove that under normal operation (below clipping), the differences between two or more amplifiers are inaudible to the human ear.
The test has been taken by thousands of people but as of this writting, nobody could with certainty (meaning, excluding pure luck) distinguish between different amplifiers.
Although I believe amplifier are not only sound quality (design and robustness of build are important factors as well), paying extra thousands for “better sound quality” should not be a decision factor. There are some good $600 amplifiers 1 out there.