I love the sound and feel of flats on a P bass. For me though, they don't work with the band that I currently play with and I find that zingy rounds are a much better fit.
My fretless which only gets played at home, is wearing a set of status half wounds, as recommended by discreet. I am very impressed, not only with discreet but also the strings . Super comfortable to play, with a warm, bright non muddy sound.