On the used market I see two versions of the Digitech Jamman looper. One has two large rubber pads for switches and the other looks very similar except instead of the rubber pads it has four switches. The two extra switches say up and down. I think the difference is that the four switch one is stereo, but the up and down buttons make me think there is more to it than that. Does this one layer more loops simultaneously? Are they both suitable for bass? The four switch one is called a guitar looper.