I've said several times that if I take the output from devices claiming to produce standard level line output, the M10 needs to be set at about 4. If you set it above that, then the output from the device feeding the M10 would have to be set to produce less than a standard line output level. My recommendation would be to use 4 if connected to unknown equipment for the first time, then adjust from there in the light of experience. You can do your own tests simply by connected the M10 to every device you can find around the place and see whether 4 works for you with them.