I can confirm on Kenwood DNX6180 the device is without SD card slot so maps must be updated via USB only.
so you need these files to update the map on your device:

GarminDevice.xml - file generated by the Kenwood device
gmapsupp.img - map file can be unlocked
gmapsupp.gma - no matter from where - it must be there
gmapsupp.unl - no matter from where - it must be there

I personally updated to the North America 2014.40 and it always said that the map is no compatible when I didn't had the UNL or GMA files.