Thanks for reply! I'm not so sure it's an open and closed case.
You see the slow proc from
https://wiki.project1999.com/Wavecrasher and bard snare/slow song do stack. I'm guessing the higher % slow is on the mob but I haven't / can't parse this myself.
On the flip side, I have tested the snare proc from
https://wiki.project1999.com/Tserrina%27s_Whip and bard snare/slow song. They don't stack. I guessing b/c
https://wiki.project1999.com/Muscle_Lock_I is just a snare and no damage.
So I think if the song (or proc) has another component they will stack.
Bard slow
https://wiki.project1999.com/Angstlich%27s_Assonance has dot and the slow/snare have other effects so the plot thickens.
Does anyone have a MEGA-equipped bard, or friend with willsapper, that they can test out if they stack?