MES-8 Midi Ports, USB DEVICE and USB Host

I've tried powering the MES-8 via the USB host port, but wondered whether this overrides any Midi In messages received on the Midi In and Out 5 PIN DIN connections or the midi host connection. Can anyone describe how these interact?

Currently if I have the USB Device port connected the MES-8 can't be switched via the USB Host or the DIN Ports. The switch positions are transmitted on the DIN out if they are changed manually.

Any help is much appreciated.