I cannot understand why anyone would say that Callas' voice after 1953 was an
embarrassment. She certainly gave some of her most outstanding performances
between 1953 and 1959 (when real vocal problems set in). Her Sonnambula from
La Scala in 1955 and Cologne in 1957 as well as her Lucia from Berlin (also
in 1955), the Dallas Medea (1958) and her La Scala Violetta bear witness to a
voice that was still in pretty phenomenal shape. Her recordings of Norma,
Ballo, Forza, Trovatore, Traviata, Boheme, Rigoletto, Sonnambula etc. are
pretty astounding, both from a vocal and interpretive perspective. And her
Dallas rehearsal of "Qui la voce" recorded in 1957 has no competition as far
as I am concerned.
I love the dark, plummy sound of the pre-1953 Callas, but I wouldn't write
off her subsequent performances either.