I too am having this issue. I bought a new unit today from costco and just started to notice the idle hum. I thought it may have been coming from the source, but even with the 3.5mm unplugged it still hums. It also appears to be unaffected by the volume knob and only goes away when the sub knob is turned all the way down, but doesn't get any worse when turned up. I have tried 2 different surge protector power strips and also directly into the wall with no apparent difference.
So far I am loving these little things, but I hope this problem can be fixed.