Srimad Bhaktivedanta Narayan Gosvami Maharaja requested his devotees in the following manner:
 
“I want you to preach my mission just as the disciples of Śrīla Bhaktivedānta Svāmī Mahārāja preached. His disciples preached everywhere and distributed his books. I also want you to preach everywhere through books. Distribute his books and my books.”

“Preach my mission, engage in book distribution, inspire new devotees, and collect and inspire new people to be devotees.”

“I think that my disciples are not doing as well as his. During his manifest stay in this world, young ladies used to go to airports, schools, colleges, and practically everywhere else. Somehow my book distribution is going on, but not like it was during his time. His disciples were not at all shy; they would go anywhere. My disciples, lady and male devotees, should also preach like this.”

—from the ‘Soul of Book Distribution’ published by Gaudiya Vedanta Publication

Srila Bhakti Raksak Sridhar Gosvami Maharaja glorified preaching in the following way:
“Preaching is sankirtana. Preaching is real service to Krsna, and not just counting beads. Service. Because Mahaprabhu and Gurudeva order it, I have to chant the Name, counting the beads. I must do my duty to the mallika, the string of beads.”The mallika should not fast.” That was the word of Prabhupada.
——-Adopted from ‘Follow the Angels.’
 
A.C. Bhaktivedanta Swami Prabhupada wrote:

“Preaching is the best service to the Lord. The Lord will immediately be extremely satisfied with one who engages in this service of preaching of Krishna consciousness. This is confirmed by the Lord Himself in Bhagavad-gita, “There is no servant in this world more dear to Me than he, nor will there ever be one more dear.”

—from the purport of Srimad Bhagavatam 7.6.24

Srila Bhaktisiddhanta Sarasvati Thakura Prabhupada on preaching:
 
“He appealed to all living beings and said only this:

yāre dekha, tāre kaha kṛṣṇa-upadeśa
āmāra ājñāya guru hañā tāra’ ei deśa

He ordered everyone to become a preacher.

—Excerpt From: Srila Bhaktisiddhanata Sarasvati Prabhupada. “Vaktrtavali: Garland of Divine Discourses.”

Srila Bhaktisiddhanta Sarasvati Thakur Prabhupada has distinguished us from other Vaisnava sects in regard to preaching work, which symbolizes the more munificent quality of Gaudiya Vaisnavas. The words of Srila Sarasvati Thakur Prabhupada are as follows:

“If we take up the practical activities of the Madhva and the Gaudiya Vaisnavas for the purpose of such comparison, we find that the former put themselves under a severe reserve in their propagatory methods, whereas the latter are vigorously proselytizing [preaching].”

—Adopted from ‘A word to Madhava Brethren’ in The Harmonist

from the second issue of ‘Engineering Your Inner Self’.