Here is my take on Stacking Songs.
The songs that your bard is singing and stacking are not the songs that he
is singing in the Taverns. He's not sitting down by the fire and
entertaining the patrons with Chant of Battle or worse yet Chords of
Dissonance. Those are songs that have special qualities to enable him to
survive the trip to his next gig. The songs that he sings in the inns would
be songs he learned before he was given his Bards Guild Membership. The
songs you have to buy are magical and are sung for the magical effect, not
for the entertainment value to the listeners, so when you have achieved the
desired effect you proceed to another magical verse for another effect.