Just a note to add to the discussions going on about bal/unbal etc. Ground _loops_ are generally exactly that - loops that, in the presence of a 60/50Hz magnetic field, function as a 1-turn transformer, picking up hum current which then pops up as a voltage due to stray resistances. If you can simply reposition cables so that the loop is no longer there, you will often rectify the problem without further ado. Keep cables close to each other, basically. For this reason, I always twist left and right interconnects in a stereo system, for instance. It's not always quite that easy when the loop includes two or three bits of equipment and the house wiring too, but a bit of thought will often crack it.
Rich
