Jakeman, this would make sense if the 600 was actually running. I have it turned off and unplugged. When I remove the Line In coax that is coming form my 2nd 350, the SMS-1 graph jumps and pegs from 30-50 hz or so. None of the subs are running through the sms-s, just watching the graph.
As soon as plug the coax back in, it tames down, I still have the 40hz hump.