From the Diaries of Count Harry Kessler (Laird M. Easton, ed., Knopf 2011), pp. 504-05:
Berlin, February 7, 1911. Tuesday.
Lunch at Frau von Rath’s with Siegfried Wagner, the Hills (the American ambassador and his wife), the painter Zorn, and Fraülein von Olfers. Siegfried brought up The Rosenkavalier, the poverty of its thematic inventiveness. He prefers Lehár or Leo Fall ten times better, etc. I defended Strauss very calmly whereupon Siegfried laughed somewhat hoarsely and said, “No, my darling” (he called me “his darling”!) “then I must forbid you to come to my concert.” He is a formless, clumsy creature, without any refinement but good-hearted. He is enamored of [Hofmannsthal’s] Oedipus, the first thing by [theatre director Max] Reinhardt that has pleased him. In this case he didn’t always notice what was lacking, as with Strauss, no nuances, no richness. Aesthetically he is a peasant.