I've been playing electric bass in a bluegrass band for 20+ years. I prefer flatwounds, either D'Addario Chromes or LaBellas, mainly because there's less finger noise. When you're playing against acoustic instruments, with no drums, you want the bass to be warm and gentle. Let the banjos and guitars be twangy and bright!
My favorite amp for playing bluegrass is a little Carvin PB150. It's a small solid state amp with a single 10" speaker. Clean, simple, and portable. I prefer the 10" speaker for bluegrass, because it doesn't have too much bottom end. My fretless bass with flatwounds played through the 10" speaker sounds more like an old Kay upright. A bigger amp with a 15" speaker tends to sound more like an electric bass.
The other key thing is your playing style. Pluck the bass up near the end of the neck, keep the amp volume low, and pluck the bass hard. And play fewer notes! Bluegrass lines usually want to be simple and precise.