I've never been happy that we have a full statement of the problem in this website. Elayne's article On Pain Pleasure and Happiness is good, and I have my collection of cites in the Fullness of Pleasure article.
At the moment the only cite I would put here is one I always thought was the most clear - from Cicero in a moment of honesty when he was attacking an Epicurean:. That the Epicureans held "that nothing was preferable to a life of tranquility crammed full of pleasures."". Which means to me that the situation we find ourselves in when all pains are gone is well described not as being some strange state of transcendance but what ordinary people can well understand as "crammed full of pleasures" (of any kind and combination, both mental and physical).
Cicero, In defense of Publius Sestius, 10.23: “He {Publius Clodius} praised those most who are said to be above all others the teachers and eulogists of pleasure {the Epicureans}. … He added that these same men were quite right in saying that the wise do everything for their own interests; that no sane man should engage in public affairs; that nothing was preferable to a life of tranquility crammed full of pleasures. But those who said that men should aim at an honorable position, should consult the public interest, should think of duty throughout life not of self-interest, should face danger for their country, receive wounds, welcome death – these he called visionaries and madmen.” Note: Here is a link to Perseus where the Latin and translation of this can be compared. The Latin is: “nihil esse praestabilius otiosa vita, plena et conferta voluptatibus.” See also here for word translations.