I bought these a few months back, and I've noticed there's quite a hum when the Marantz is switched on but there's no sound (or when it's low). I generally leave the Mission sub switched on, and that's fine. What could it be? Could it be a mains problems? Does the Marantz know (or need to know?) that the sub is active? Any advice would really be appreciated. It's not really bad, just a little irritating when I notice it.....! Thanks.