Is there a correlation between sukha and passadha? Is there a correlation between sukha (the 4th component of the jhanangas) and passadha (the 5th component of the sambojjhanga)?
When Piti arises and you stay equanimously watching the arising and passing knowing its impermanence (with no attachment or craving to this feeling) then Piti will fade away and Passadhi will arise. When Passadhi initially arises you may think your meditation is regressing as you loose Piti and also board after a few hours of Passadhi.

Piti is a stage which is conducive to tame Sankara resulting from craving.

Passadhi is a stage which is conducive to tame Sankara from Ignorance or Sanna.