So going back and forth with the Peachtree Audio guys (very helpful BTW) they determined there was a current leak across the USB ports between the Mini and the Grand. If I used my Apple Cinema monitor, which has a grounded plug as well as needing one of the USB ports to function, the problem went away. It also was able to be resolved by running a 3.5mm to RCA cable between the Mini and the Grand. So it comes down to a grounding issue. They offered to do a simple mod for free to resolve it permanently. Or I could just keep the 3.5mm to RCA in place....but that ties up an input on the Grand.
Haven't decided what to do yet. But good to find out there are a couple of possible solutions.