this is how haste stacks.
1 item
1 spell/song
1 effect
1 overclock bardsong haste
that is it. nothing more, unless the effect is now counted as a spell/song.
take the best haste from any item you have that has haste, that is only haste that will count for items, same for songs/spells/effect, then you can add the small group of bard songs that are overhaste songs.