With some sounds you may get an irritating bit of distortion that seems impossible to fix by just lowering the main volume... that is because the clipping is happening in the Curtis chip, not after it where the volume parameter is active... in the manual it says that with high resonance, it may clip, so try lowering the resonance if any, and see if that helps... otherwise there really is no way to lower individual oscillators volume because the volume of each is controlled with a mix parameter and not individual volumes (which is a hardware thing in the chip)...
You can control the volume if you use only one oscillator though, by setting one oscillator to "off", and then control the volume of the other with the oscillator mix parameter, which may help if the program only use one oscillator as an audio source...
Other things you do have control over though are Sub and Noise...
Another source for clipping may be the FX engine... try adjusting the FX mix parameter to see if that helps a bit.