B string flatwound muddy?

[BoomingBass]

I got a Douglas 6 string fretless bass:
Douglas WOB 826 Fretless Nat at RondoMusic.com

Anyway, it came strung with flatwounds. I thought this was very nice especially since I normally prefer flats. The problem is the B string. I have never owned a five or six string bass, so I may just be stupid. It is practically inaudible acoustically, and very quiet amped. I boosted the bass and the preamp, but the sound is mud and has horrible sustain. This is very different than the other strings. The E string has the nice flatwound growl I know and love without any EQ. The B string also rattles when any other note is played but B. That's probably just because I've yet to set it up, though.

Opinions, advice?

[FunkMetalBass]

I'd say put it through a setup and adjust pickup height before making and judgments about the strings or bass. Granted, I don't doubt the strings are probably not the best (and many sets with B-strings are notoriously floppy), but it's amazing what difference a setup can make.

[Epitaph04]

Definitely give it a setup. Usually when I play a 5 or 6 string bass the B is pretty damn loud. It shakes stuff and whatnot.