Ok since no one else wants to go there............
if using a metal case the pot is already grounded if the case is at circuit ground , which it would normally be or why use a metal cabinet ?
but use a plastic or wood case and there is no ground connection to the pot,or for that matter any other part of the device in question
now if you look at that same pot you will see the biggest hunk of ungrounded metal in the entire amp,even the tiny guys are still bigger than any other metal part
still with me ?
this big ass hunk of metal is like an antenna for any garbage that happens to be floating around your area or in the actual circuit itself, not to mention the stray capacitance potential which can play havoc with wideband circuits
so botom line this pot acts like a free agent rather than part of the team