In order to get that 'GrUnGe' sound, you should use a bass with humbucker pickups, and a distortion pedal as well. The bass most frequently used by Krist Novoselic was the Gibson RD Artist, the 'black sheep' of basses. It's pretty rare, but if you find one, buy it! The pedals used most frequently by Krist was the Big Muff (fuzz) and the ProCo RAT, since they don't take away that much bottom. Now you're ready to rock on!