What do you mean "keep them from clipping"? What mic are you using?
W/ Presonus preamps (like many lower gain preamps) one can have a hard time finding a sweet spot w/ certain mikes. But basically it is learning to use your tools.
However, it shouldn't be that hard to get a vocal sound -12 dB or even lower. The great thing about digital is the noise floor is so low you can raise the recorded level. If you are down in the lower -20 db peaking range you should be able to raise the average level w/o peaking. THis goes for vocals and all tracks.
The gain in SONAR should be all 0's on faders and such, exept for your already recorded material which should be leveled or rough mixed. The correct gain is acheived by the preamp in the Presonus (see above pp).
Using a soft comp etc. won't really do anything to your recorded sound going in - it only effects the recorded sound. It is mostly used to flatten and fatten the track and is a whole 'nother topic. Play around w/ it during mixing, but don't feel obliged to use it. W/ soft comps/eq, it is easy to overdue. They are there, why not use them? Because you might not need them. Experiment, but don't commit until you get a better grasp of what you want to do.
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