My Mark III has a ground-mode switch. For what it's worth, I've never noticed any difference in sound or noise regardless of what setting it's on, so it might not even be hooked up anymore. Anyway, isn't a ground switch like this a sign that the amplifier has the so-called "death cap"? Should I be looking into this and confirming that it's been changed to a more modern grounded design? It may have been done already, the power cord is a 3-prong grounded one.