If you're looking for some ballads or slower to mid-tempo songs with strong bass here are a few i recommend. They also seem to be some of my favorite ayu ballads ♥. In these type of songs the bass lines are practically solos during the parts where not a lot of instruments are playing...
End roll (the bass really has a lot of notes in this one!)
Far away
Endless sorrow
Daybreak "HAL's Mix 2002"
HANABI~episode II~ (the verses have hecka bass)
Liar (same as above)
GUILTY (does some things other than the bass note)
+ fated... already been said by Coelacanth, though, but it's one of my favorites
v that too!