I wouldn't call ataraxia the "main constituent" but I may go so far as to call it a necessary condition but not a sufficient one.
Questions to find more clarity:
-- Is there a physiological need in some people to seek out more "tranquility" because they are very sensitive to stimulus and easily disturbed by sensations of sounds.
-- Is "tranquility" a need which only arises in dependence with a given environment? Some environments are so tranquil that there is actually a much greater need for new sensory stimulation to allieviate boredom.
-- Is the basis of "tranquility" suggesting that Epicureans will live in out in the country-side and away from noisy cities? No need to find a "cave" to live in, but for some there is a need to move out of a city and live in the country -- at least for those who have overly-sensitive physiology.