I've noticed that before on the studio cut.
HERE is a link to her performance on the Tonight Show w/ Jay Leno, where it seems it's more just the guitar doing the grind, though the bass player looks to be using an Eden Navigator pre, so he may very well be getting some grind from that.
edit: around the 2:40ish point of that live video, where the guitar and bass do a climb out of the bridge, the bass tone is a little more defined, and sounds pretty clean, IMO.