Given the inherently scooped nature of both the B3k and flatwound strings in general I personally don't think they are an ideal combination. Roundwound strings have plenty of harmonic content to spare so the B3k focuses it nicely, whereas worn-in flats are focused to the point that scooping more seems to take away too much for my needs (fresh flats are an exception, but most people don't constantly change them out!).
Not to say that there weren't good tones in there when I used that combo, but I think flats pair better with pedals with fuller added midrange- I found the Darkglass Vintage to be a much better match personally. That said, I did find decent tone with the Raw switch selection and a fair amount of clean blended in (for reference, on my Darkglass Vintage I don't use any Clean blend at all and much prefer the tone).
G&L Club Member #10