Does Willsapper Energy Sap proc stack with bard slow songs?

[DeathsSilkyMist]

I don't think you need willsapper specifically to test this, unless willsapper's debuff is somehow unique. You simply need to test if bard slow stacks with other slow spells such as Turgur's Insects. I am pretty sure they do not stack, since both are still considered to be spell slow. It is the same concept as spell haste. The only bard haste that stacks with other forms of spell haste is Evaj, because it is specifically V2 haste. Most bard haste spells are not V2 haste, which means the game simply considers them as a haste spell, no different than something like Alacrity. Whichever haste is better will take priority.