If you can use the spdif digital out you can negate a lot of the noise floor and just deal with the synths coming in.
Nothing wrong with using 0db for your master out as long as you leave yourself enough headroom to not create digital distortion/clipping.
I personally use -12db/-6db depending on what I am doing, and mostly so I can get a little bit more sound through my headphones.
Detomasso, are you using inherently noisy synths, or maybe have a ground loop problem? I usually don't hear too much noise that I have to deal with when running my Voyager in. Maybe I'm not being picky enough though

. Usually it's my turntable that has way more noise, but it probably needs to be repaired :/.