[...] In Vienna recently I got a renewed glimpse of the quality of our singers—of our tenors in particular—which very much depressed me. I was forced to ask myself once more how it was I conceived the notion of writing all my chief spiritually significant rôles for the tenor voice! But my most remarkable discovery was that these people have no real voices, and I could only make this plain to the kapellmeister and director by pointing out that their voices are incapable of euphony. "Just listen," cried I, "how Herr Müller, for example, brings out that 'Thanks be to thee, my lovely swan!'" That was a bombshell! Then I went to great trouble, in particular forbade the singer to take breath so often, and attempted to get at least a seeming euphony by all possible evenness and a consequent subduing of the naturally harsh tones. Yes! Let people try to get that effect quickly if they can!! This charming pulsating "smile" in the voice that costs no pains and yet captures and enchants the soul! It is a thing unknown to-day! In Tichatschek it was a natural gift, vocal euphony. The last time when I came upon Unger at the Nibelung headquarters and he finally sang from Siegfried, it was principally the great industry he had expended on his studies which decided me in his favour. But what gave me real hopes of him (and of you, dear friend) were the mere singing exercises in solfeggio that I overheard from outside before I came into the room to you. I hope Unger will not on any account neglect—these mere vocal exercises, because he thinks little of them, does not yet see their bearing on his part, or the like. This is what I ask of him, and I am glad to see him justifying the good opinion of him which led me to embark on the lawsuit, feeling that I was concerned not only for the "Siegfried," but for Unger, the singer, himself. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 683