Is there a correlation between sukha and passadha?
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.