In his 1946 Foreword to Brave New World, Aldous Huxley writes:

“This really revolutionary revolution is to be achieved, not in the external world, but in the souls and flesh of human beings. . . . Sade regarded himself as the apostle of the truly revolutionary revolution, beyond mere politics and economics . . .

. . . —the revolution of individual men, women and children, whose bodies were henceforward to become the common sexual property of all and whose minds were to be purged of all the natural decencies . . .

. . . Sade was a lunatic . . . The people who govern the Brave New World may not be sane . . . ; but they are not mad men . . . It is in order to achieve stability that they carry out, by scientific means, the ultimate, personal, really revolutionary revolution.”

— Aldous Huxley, Brave New World, 1994, Flamingo, London. Originally published: 1932.

Huxley also refers to the Marquis de Sade and the “ultimate revolution” in a letter to George Orwell:

The first hints of a philosophy of the ultimate revolution — the revolution which lies beyond politics and economics, and which aims at total subversion of the individual’s psychology and physiology — are to be found in the Marquis de Sade, who regarded himself as the continuator, the consummator, of Robespierre and Babeuf.

–from his letter to George Orwell in which he compared the likelihood of the futures presented in 1984 and Brave New World
http://www.lettersofnote.com/2012/03/1984-v-brave-new-world.html
Written 21 October, 1949 from Wrightwood, California.
Original source: Letters of Aldous Huxley

Secondary references:
https://canadianliberty.com/?p=16929
https://canadianliberty.com/?p=9717