What's your budget?
Cheap options: Behringer BDI21, Digitech Bad Monkey, DHA VT1
More expensive: VT Bass, Xotic BB Preamp
Mega expensive: Darkglass B3K
I wouldn't go for a boost like the Phat Phuk B with the OTB, that amp sounds a bit farty when you boost the gain into it, it's no AD200B!