For the noise and oscillations, I am not sure what you are referring to; noise can indeed happen in case of a ground loop and this can be overcome (see
http://forum.arturia.com/index.php?topic=90189.0).
If you are referring to background noise, it is true that with such a huge dynamic (9 waveforms + noise can be mixed into each filter, actually 14 waveforms counting the ultrasaw), one has to take more care of gain staging than with much simpler architectures such as a MiniBrute's. Also compared to the MiniBrute (or other synths which VCO level can be high enough to drive the filter), note that some of the VCO level gain has been "transfered" to the drive feature of each filter so they wouldn't drive that much when Drive is 0, even with a VCO in at full level.
You probably already know that, but just in case, to minimize background noise you might want to follow this rule: the closer to the sound source, the higher the level, up to the limit of distorting the next stage too much to your taste. This way the later gain stages (e.g. phones level) shouldn't be too high in level -and noise sould be reduced. Also if you are not using one of the filters you might want to set its level to 0!
Oscillations.. are you talking about the small VCO bleed in the Steiner filter ? The good news is that this bleed is actually being filtered by the filter, so if you are using the SP filter you should be feeding it with sound sources that should be way louder than this bleed and not be bothered by it. If you are using it with external-in only, you might want to set the VCO pitches to extreme settings so you don't hear the bleed anymore. If you are using it for self oscillation playing only... you must be a masochist
as the Steiner filter inherently does not track well at all. Finally if you are not using it, it might again be a good idea to set its level to 0.