I don’t think you can stream and scan using firefly over the network.
It does work using explorer as the network connect is implemented in explorer, but not in firefly trying to access it as a local ‘dir’.
However, why don’t you run firefly on the second (file)server?
Firefly is very lightweight, and broadcasts itself over the network.
So instead of moving the files, why not move the entire ‘setup’ including music. Also, I believe you can configure fireplay to locally connect to the second server instead of localhost, so that should work too.