Les Grandes Orgues Cavaillé-Coll

In 1881, Louis Vierne ( a boy of 11 years old) visited for the first time the mass at Sainte Clotilde and heard Cesar Franck play:
I clearly remember that Mass. We arrived early and I waited impatienty. The organ played a mysterious prelude, quite unlike any I had heard at Lille; I was bowled over and became almost ecstatic. There was more to come at the Offertoire, where the master had more time; the theme so unfamiliar, yet so attractive, such rich harmonies, such subtle figurations, and a pervasive intensity that astounded me. I reveled in such delights and wished they would never end. We listened to the Sortie right up to the last note; it was a long paraphrase on the "Ite missa est," full of lyrical flights of fancy that conjured up for me heavenly visions of processions of angels chanting ‘Hosanna’. At the same time, certain melodic progressions and harmonies made me feel uneasy and sensual at the same time. I could not hold back my tears. I knew nothing; I understood nothing; but my natural instinct was violently shaken by this expressive music echoing through my every pore. Faint premonitions of the true meaning of music arose in me. I could not express it in precise terms, but when my uncle asked me what I had felt and what it had done to me, I replied ’it's beautiful because it is beautiful; I don't know why, but it is so beautiful that I would like to play such music and die immediately afterwards’. My aunt was alarmed at my reaction and took me home in a carriage, my legs refusing to carry me. She discussed the matter with her husband and expressed concern. Wisely he reassured her, convincing her that, almost inevitably, this reaction proved my future lay in music.
Reference: Louis Vierne, Journal (excerpts), Cahiers et Mémoires de l’Orgue, No. 135bis (Paris: Les Amis de l'Orgue, 1970) 129 (English translation found in : Louis Vierne- organist of Notre Deame cathedral. Rollin Smith, Pendragon Press Hillsdale, N.Y. 1999).

Maurice Duruflé
The quality of the Récit was something of a miracle. Undoubtedly several technical reasons contributed to this: the dimensions of the swell-box; the responsiveness of the shutters; its location at the back of the organ case; the large sonorous space surrounding the box on all sides, giving it an extraordinary resonance; the acoustics of the church; and above all, the genius of the builder. These factors produced a miracle.
Maurice Duruflé: Mes souvenirs sur Tournemire et Vierne L'Orgue 162: 4, 18

André Marchal
In 1912, the date at which Ch. Tournemire invited me to touch it for the first time, this organ had had maintenance only once since the death of C. Franck. At the beginning of the century, Mutin added a coupler to the Swell which did not exist yet. This explains that in written passages on the Swell with pedal, Franck doubled the last with the left hand. This organ is considered the most poetic of all the Cavaillé-Coll instruments. It was harmonized by Gabriel Rimburg, being the most artistic of the harmonistes of Cavaillé-Coll. Its sound is characterized by a great poetry of the foundation stops and an extreme lightness of the reeds. The great organ has a composition similar to that of many other organs of that time, only the lack of a Cornet and the clarity of the Mixture of VI ranks, brighter than most mixtures of Cavaillé-Coll, is characteristic for this organ.
The pipes of the GO are arranged on each side of the positive, the latter being located in the front. The positive, almost as important as the Great, has a clarinet which tone color and power allowed Franck to use the full closed Swell (funds and reeds) as accompaniment (the Grande Pieèe Symphonique - Andante). The impact of the positive is still improved by its location in the front. Responding to these two, a small Swell of 10 stops is located behind the positive. The poetry of this keyboard was unique: the smoothness of the foundation stops, the mysterious Voix Céleste and Voix Humaine, the clarity of the Haubois and the exceptional light Trompette and Clairon, they all together allowed this keyboard to equilibrate with the other two. The Swell produced an exceptional effect and allowed a pianissimi of such a restrained character that, when mixing the foundations and reeds, the latter disappeared almost completely when the box was closed. This explains why Franck often retained the reeds of the Swell in his registrations: it was sufficient to close the Swell because then all of the foudatinos dominated. This is hardly ever seen in other organs, even those of Cavaillé-Coll, and that is why the registrations of Franck as he wrote them down can almost never be applied literally.
(Source: disk CD Erato tribute to André Marchal 1958/1994)