First of all, I am a flatwound strings fan.
I have them on 6 of the 8 basses in my collection (including my upright, which is kinda a given), and other than new flatwound strings I am trying out, they all have Sadowsky flats on them.
I've tried a number of flatwound strings from all the major makers.
These just tend to be the best of all worlds.
They feel great. Not low tension (higher than TIs), and not too stiff (my major beef with Chromes)
They sound great, both fingerstyle and with a pick (nice snap when played with a pick).
They are not the cheapest, but not the most expensive.
And they give you that sought-after vintage tone without being muddy.
Sadowsky Flatwound (lights)
Sadowsky Black Label Flatwound Bass Strings - 4 String Set