Im not sure -10dBv and +4dBu voltage levels apply here as the ART Dual RDB has no settings for that.
So that leaves the Interface or the Microphone.
To be honest I didn't think it was possible to Re-amp using the Saffire 212 unless..
You are doing the recording live (One DI goes to track 1 / One Mic signal goes to track 2).
Not listen to dry tracks later and amp them to a source.
you can try...
1- make sure impendences are matched
2- Try the phase switch
3- Check your routing
You need the Focusrite MixControl software to do internal routing but I don't believe your interface doesn't support internal routing.