^ ah- I got thrown by your mention of blind tests (which usually show lack of audible difference) and the once-sentence summary of the study "When compared evenly, the sonic differences between amplifiers operated below clipping are below the audible threshold of human hearing". Which wouldn't apply to some tube amps because they add audible distortion, I agree. But some people do like the sound of added harmonics and some headphones are built far too bright (Beyers, Grados) and tube amps can de-emphasize their treble. In the first case some pleasure can result from this particular type of distortion and in the second the distortion can make flawed headphones more faithful to the original recordings.