The last major composer known to have seen the Telharmonium was Edgard Varèse. It was after the bankruptcy, but the seed had been planted years ago by Busoni. In his 1907 classic, Sketch of a New Aesthetic of Music, the eminent composer-pianist Ferruccio Busoni had set down a miscellany of thoughts on the nature, spirit, and role of music. The lengthiest section of this monograph was a tirade against the tyranny of equal-tempered tuning. Nature, he claimed, had created an infinite gradation of tones. Yet music was locked into 24 keys which were really only two keys, major and minor, the rest being mere transpositions. Furthermore, since major and minor represented two faces of the same whole, there was left really but a single meager key for composers to work with. Busoni then began to tinker with the arrangement of seven tones in an octave, reporting that he had managed to construct 113 different seven-tone scales, using the twelve tones of equal temperament. The wealth of new melodic and harmonic possibilities was obvious. 
Busoni then proposed a one-sixth tone scale, which would allow the formation of a one-third tone scale within the framework of the familiar semitone scale. Enter the Telharmonium. The article New "Music for an Old World" by Ray Stannard Baker, in the July 1906 issue of McClure's Magazine, had reached Busoni while writing his essay. Thaddeus Cahill's invention solved the problem, wrote Busoni, of how such scale divisions were to be produced. The infinite gradation of the octave could obviously be produced on the Telharmonium by simply setting a lever. "Let us take thought," he exulted, "how music may be restored to its primitive, natural essence; [...] let it follow the line of the rainbow and vie with the clouds in breaking sunbeams." 
Edgard Varèse had read Busoni's little book in 1907, when he was 24. It made a great impression on the young composer. Varèse moved to Berlin the same year and soon became friends with Busoni. They avidly discussed the new electrical instruments, in which Busoni was very much interested. Varèse was surprised, in view of Busoni's interests in such unrestrainedly unconventional theories, "to find his musical tastes and his own music so orthodox."  This was evident in, Busoni's American tours of 1910 and 1911. He played only the traditional repertory to gloriously enthusiastic tributes and apparently made no effort to see the Telharmonium while in the United States. 
Varèse arrived in America on December 29, 1915.  He was then greatly concerned with the need for new instruments and expressed these ideas in a newspaper interview several months later.  It must have been around this time that he learned that the Telharmonium was still in operation on West 56th Street, even though all thoughts of commercial service had been abandoned. He went to hear it.
[Busoni] was very much interested in the electrical instruments we began to hear about and I remember particularly one he had read of in an American magazine, called the Dynamophone, invented by a Dr. Thaddeus Cahill, which I later saw demonstrated in New York and was disappointed. 
 Ferruccio Benvenuto Busoni, Entwurf einer neuen Aesthetik der Tonkunst, Leipzig and Trieste, 1907, tr. Theodore Baker as Sketch of a New Aesthetic of Music, New York: G. Schirmer, 1911, reprinted, New York: Dover Publications, 1962, 89-92.
 Ibid., 93-95.
 Edgard Varèse, "Ferruccio Busoni A Reminiscence", Columbia University Forum, Spring 1966., IX:2, 20.
 "Busoni and the Chickering", Musical Age, January 15, 1910, LXVIII:12, 214-215; "Busoni and the Chickering", Music Trade Review, January 28, 1911, LII:4, 53; "Touring the United States with Busoni and Garden", Music Trade Review, June 24, 1911, LII:25, 49.
 Fernand Ouellette, Edgard Varèse, tr. from the French by Derek Coltman, New York: The Orion Press, 1968, 44.
 Ibid., 46-47.
 Louise Varèse, Varèse: A Looking-Glass Diary, Vol. I, New York: W.W. Norton and Company, 1972, 50; cf. the publication of this text in Columbia University Forum, Spring 1966, IX:2, 20, in which the words "and was disappointed" were omitted.
Da Reynold Weidenaar, Magic Music From the Telharmonium, The Scarecrow Press, Metuchen, N.J. - London 1995, pp. 253-254, 261.