That is a ground switch. It controls the polarity oof the ground, and whether it is connected or not. It hleps get rid of 60 cycle hum (oftebn heard when bad/dirty power is present). it is a nice little feature, so you don't have to plug your amp into an adapter.
So if set on 0 then I am not grounded...and I can have + or - ground. I am guessing that it is prefered to be grounded, is there any difference between a + or - ground if all other things are equal (and if both work equally well?)
If your amp has a 3 prong plug, then you are always grounded regardless of the switch setting (assuming the outlet is grounded correctly). And if you have a 2 prong plug, then you are never grounded regardless of the switch setting. The switch is just connecting a capacitor from chassis to either of the AC lines, or neither of the AC lines in the 0 position. It helps with high frequency noise in some situations (fluorescent lights, etc.). To set it, plug in your amp, plug in your bass, and touch the strings and listen for a "pop" from your speaker. Flip the switch and try again. In one of the positions, there will be less or no "pop", and that is what you want. Chris