"Isaiah's Commission When God was about to send Isaiah with a message to His people, He first permitted the prophet to look in vision into the holy of holies within the sanctuary. Suddenly the gate and the inner veil of the temple seemed to be uplifted or withdrawn, and he was permitted to gaze within, upon the holy of holies, where even the prophet's feet might not enter. There rose before him a vision of Jehovah sitting upon a throne high and lifted up, while the train of His glory filled the temple. Around the throne were seraphim, as guards about the great King, and they reflected the glory that surrounded them. As their songs of praise resounded in deep notes of adoration, the pillars of the gate trembled, as if shaken by an earthquake. With lips unpolluted by sin, these angels poured forth the praises of God. “Holy, holy, holy, is the Lord of hosts,” they cried; “the whole earth is full of His glory.” [See Isaiah 6:1-8 .] GW 21"
"In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple. Above it stood the seraphims: each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly. And one cried unto another, and said, Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory. And the posts of the door moved at the voice of him that cried, and the house was filled with smoke. Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the Lord of hosts. Then flew one of the seraphims unto me, having a live coal in his hand, which he had taken with the tongs from off the altar: And he laid it upon my mouth, and said, Lo, this hath touched thy lips; and thine iniquity is taken away, and thy sin purged. Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? Then said I, Here am I; send me."
Ellen White directly references Isaiah 6:1-8 in this passage.
"The seraphim around the throne are so filled with reverential awe as they behold the glory of God, that they do not for an instant look upon themselves with admiration. Their praise is for the Lord of hosts. As they look into the future, when the whole earth shall be filled with His glory, the triumphant song is echoed from one to another in melodious chant, “Holy, holy, holy, is the Lord of hosts.” They are fully satisfied to glorify God; abiding in His presence, beneath His smile of approbation, they wish for nothing more. In bearing His image, in doing His bidding, in worshiping Him, their highest ambition is reached. GW 21.2"
"And one cried unto another, and said, Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory."
The EGW passage directly quotes and elaborates on Isaiah 6:3, describing the seraphim's worship and the future filling of the earth with God's glory.
"As the prophet listened, the glory, the power, and the majesty of the Lord was opened to his vision; and in the light of this revelation his own inward defilement appeared with startling clearness. His very words seemed vile to him. In deep humiliation he cried, “Woe is me! for I am undone; because I am a man of unclean lips: ...for mine eyes have seen the King, the Lord of hosts.” GW 21.3"
"Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the Lord of hosts."
The EGW passage directly quotes and elaborates on Isaiah 6:5, making it a clear explicit allusion.