Many sources report that Debussy's playing was delicate and subdued [Long; Vallas]. Was this his ideal? One account from George Copeland says something about Debussy's ideas on soft playing. When he met Debussy for the first time and was going to play for him, Debussy said:
The piano [...] was draped with a silk scarf held in place by a heavy cloisonné vase. I asked permission to move the vase, so that I might open the piano cover. 'Absolument non!' he replied with obvious annoyance. 'Do not touch it! I never permit that anyone should open my piano. As it is, everyone plays my music too loud.'[Nichols p. 164]
Dumesnil tells us about the instruction he received from Debussy about playing the music. Debussy said that dynamic effects should not be exaggerated:
At the crescendo leading to the climax, marked ff, he stopped at my side: 'Please do not overdo this crescendo. It sounds too dramatic; start more softly and you will reach the same effect without impairing the quality of your tone. [Nichols p. 159]
Dumesnil goes on:
Remembering his previous remarks about dramatizing, I tried to keep the middle part [of Clair de lune] moderate. But I guess I still overdid it: 'No,' he said, 'you exaggerate both the crescendo and the rubato. The latter must be done within the entire phrase, never on a single beat.' And the expression had to remain dignified. [Nichols p. 159]
Thus Debussy wanted a crescendo that was not exaggerated, to "remain dignified". But Debussy was definitely not after a monotonous sound, something he was accused for in concert critiques [Vallas]. The above quotation shows that Debussy was after more subtle effects, that would not ruin the quality of the sound.
E. Robert Schmitz also comments on how Debussy preferred the more subtle crescendos rather than exaggerated dramatic ones:
Crescendos in those days were one of Debussy's obsessions in piano playing. He liked slight crescendos, a ppp increasing into a mere pp. Such tiny changes were meaningful and important to his art. So many pianists, who play Debussy today, overlook his crescendo markings. Seeing the sign, ppp then a crescendo, they seldom bother to look for the volume mark at the other end of that crescendo. Immediately they spurt of into an fff. It is such carelessness which makes so much of Debussy's music for piano sound jerky, heaving, rather than delicately flowing and wistful, sustained, the way it was originally intended by him to be. [Nichols p. 171]
It seems that subtle nuances were important for Debussy. Dumesnil's method book contains some interesting exercises in subtle nuances. These concentrate on playing chords pianissimo, and to be able to play a diminuendo from pp and fading. He regarded this as essential for playing Debussy well [Dumesnil p. 8].
In the same book he urge more moderate fortissimo climaxes: "...in all of Debussy's compositions, keep the “fortissimo” climaxes moderate." [Dumesnil p. 23].:
E. Robert Schmitz on the other hand is of the opinion that one should not limit the dynamics in any way in the music of Debussy. He makes a point of the authority the score:
It is, then, in the works that we find the answer to his pianistic style and demands in performance, and not in hearsay reports of his “violent attacks on the keyboard” or his “constant pianissimo” playing. [Schmitz p. 35]
He points out that the works of Debussy uses the resources of the piano to the full extent, with a dynamic range from fff to ppp, and needs all kinds of nuances and articulation. Still I think Schmitz would agree with Laloy who says that the music "will not tolerate any ugliness, even intelligent ugliness." [Priest p. 109]. If the music of Debussys sometimes demands a full sonority, so must the sound never be ugly.