After a lot of tests, I found a good sound with built in Clip+ headphones, that are famously not good sounding.
In Settings-> Sound Settings -> Equalizer -> Graphical EQ, we have 5 band to change:
BAND | Gain | Frequency | Q
LS -2,5 50 1
PK1 -8 140 0,7
PK2 -2 800 0,9
PK3 1,5 3500 0,9
HS 8,5 10200 1,5