My bandmate had this bass combo Peavey TNT 130 dusting at his folks place so he brought it to our garage. The amp features a "GND switch" with '+' and '-' positions. Which possition is supposed to be used and what does this switch do anyways?

Here's a pic of the amp http://lh6.ggpht.com/_d3YcwPQ2H9U/RrYZheuO5ZI/AAAAAAAAABI/AOh8mmreWMs/IMG_1067.JPG

Im 90 percent sure its about itf your amp is sitting at an angle, or if it is above or below ear level.

For example: + would mean its below you, and vice versa.

I could be wrong, but i think i read that somewhere.

EDIT: Nevermind, i think it was something else im talking about.
Gibson Les Paul Custom
Fender American Tele

Orange Rockerverb 50
Orange PPC412
It switches a large capacitor to ground from either the positive or the negative prongs of the power cord. Ideally, you should have a tech remove the capacitor it since it's not a particularly safe component. It's a very easy process and you can even do it yourself if you're comfortable opening your amp up safely.
If you do decide to continue to use it, just switch it to whichever setting hums the least.