if you can't tell the difference between a Pythagorean major third and a just one, you can't tell the difference between just and equally tempered thirds either

Very probably, Jack - although I remember being very impressed by a demonstration of the pure 5/4s on Harry Partch's chromelodeon (a reconstructed harmonium - the only of HP's instruments to feature his full 43-note octave). However when I'm playing in thirds on the harp (the 13th century Nobilis Humilis for example) I'm not aware of any imperfections as such.