I posted this early on:
The only other solutions are to add more ballad + gear on slots that currently don't have all song or song duration on them. Adding more all song gear would just keep the ballad to other song gap the same. The current gap is 27% between acc/march compared to ballad duration.
Or to increase the base duration of ballads by 27%, 32.4s.
Ballad +2 on empyrean legs is not enough to bridge the gap between other songs. To match other songs legs would have to be Ballad +3.7
I will elaborate further:
Ballad max duration comes from Carnwenhan 60s(120 x 50%), Gjallarhorn 48s(120 x 40%), Moonbow whistle +1 36s(120x30%), Fili Hongreline +1 14.4(14 se tends to round down)(120x12%), Fili rhingrave +1 12s (120 x 10%), Brisoso sillpers +3 18s(120 x 15%)
120 + 60 + 48 + 36 + 14 + 12 + 18 = 308s
It is because other songs can use the duration legs inyanga shalwar +2 (120 x 17%), but for ballad potency you must use the Fili rhingrave +1. This results in a minimal 7% difference in duration to all songs, add in the fact that for marches you get an additional +1 from Fili manchettes +1, adds another 10% for a 17% difference for marches. Add in intarabus's cape(120 x 10%) and Fili calot +1 (120 x10%) for madrigal results in a 27% difference in duration from ballad.
Basically the difference in duration comes down to the use of ballad legs or duration legs and the fact other slots have specific song +1 attributes