Ruskin at MP I:33 appears to be directly challenging the views of Reynolds in Discourse Ten of 1780:
Its essence [i.e. the essence of the sculptor's art] is correctness: and when to correct and perfect form is added the ornament of grace, dignity of character, and appropriate expression, as in the Apollo, the Venus, the Laocoon, the Moses of Michael Angelo, and many others, this art may be said to have accomplished its purpose. ( Reynolds, Discourses, p.178)
The canon to which Reynolds was appealing, and which he had helped to establish, was still the basis for judgements of the 'Elgin Marbles' in The Report of the Parliamentary Select Committee on the Earl of Elgin's Collection of Marbles (1816) (see the Friezes of the Parthenon).
For Winckelmann in 1764, as for Reynolds, the Apollo represents the 'highest ideal of art': 'An eternal spring, as in the happy fields of Elysium clothes with the charms of youth the graceful manliness of ripened years, and plays with softness and tenderness about the proud shape of his limbs' ( Winckelmann, History of Ancient Art, VI, III).
For Hazlitt on the Belvedere Apollo and the Venus de' Medici, in 1826 it is 'positively bad', and 'ill-made'.( Hazlitt, Notes of a Journey through France and Italy, p. 222). Hazlitt makes a point of contrasting it with the so-called Theseus, now known as Dionysus, from the east pediment of the Parthenon which formed a part of the collection removed by Lord Elgin, and displayed in the British Museum since 1816.
An account of the changing reputation of the Apollo is given in Haskell and Penny, Taste and the Antique, pp 148-151. A summary of the influence of increasing knowledge of classical Greek sculpture on attitudes to the work such as the Apollo is given in Haskell, Rediscoveries in Art p. 6 .detail some of the changes in the reputation of the Apollo. The Venus de' Medici to which Ruskin refers at MP I:404 has undergone an equally drastic critical revaluation.
Despite Ruskin's apparent doubts about the Apollo at MP I:33, in his reference to it at MP I:404 he appears to suggest that he still considers it a work of major importance, rather than the 'scraped turnip' which Haskell reports as the comment of French students at the end of the eighteenth century.
See Ruskin and the Italian School.