http://www1.epinions.com/content_312424107652 is what sold me on the Yaqin, but if you Google on "Yaqin tube buffer" you'll find lots of supporting comments. Maybe I'll upgrade the tube sometime but right now I'm very satisfied - even of the spec'd HD is 3-5 times as high as the amps you mentioned in your first question.
Re the audibility of HD in general, we need to remember two facts not often available: (1) high levels of odd-order HD make the sound irritating, while even-order HD simply sounds more "musical", even if there's a theoretical diminishment in perfect accuracy, but by high order I mean distortion levels of 2% or so; (2) what counts is am amp's HD at actually used rather than maximum output level; i.e., less than 5 watts - 500 milliwatts for high efficiency speakers like Klipsch or (yay!) Lowther. In fact for such speakers a half-watt feed from the amp would have you hollering "TURN IT DOWN SO I CAN HEAR MYSELF THINK!" This is not hyperbole.