2 ears and a brain, the most incredible audio analyzer in the universe.
This generally makes people angry to read, but here goes:
When anyone touts how an amplifier sounds better than other amplifiers, and does so based on sighted tests, it is more than 2 ears and a brain. One's eyes are playing a major part of the test. Floyd E. Toole has done extensive research in this matter, and The Audio Critic has done so as well.
Hundreds of blind tests have been done on tube amps versus solid state amps for decades, and there has never been a properly run test in which the test subjects could hear these differences under blind tests with one exception: A tube amp with a non linear response curve will sound different from a solid state amp with a linear response curve.
A non linear response curve has a comparable effect that an equalizer has on a system.
