http://users.ece.gatech.edu/~mleach/papers/TubeVsTrans.pdf

It involves a series of AB tests on experienced guitar players in terms of judging frequency response, ripple distortion, electrical impedance effects, and a number of other areas. It actually turns out according to this article the main reason that a tube amp sounds so much different from solid state is the different frequency response created from the speaker impedance load.