I use Cobalt flats on my fretless 5 string. They're pretty zingy and hot when you first put them on, but give them a few sessions to mellow out. They have a lot of dynamic range and feel very nice under my fingers. The midrange has tons of character and the treble retains some shimmer. The flatwound "thud" present on strings like Chromes is either significantly less pronounced or not present on the Cobalt flats, the open E was super growly when the set was fresh. Excited to see these in a few more months.
I totally recommend trying them.
I totally recommend trying them.