Not really sure what you mean. Sometimes I thicken a bass line by mixing in a little of a synth patch that's just a triangle wave, you can make a triangle wave in most synths. Sometimes I find it helps to flick the phase switch on the track in case there are phase issues between the two parts.
If it's a "regular" bass sound that you're after then Sonar comes with SI-Bass does it not? Have never tried it because I have some decent Native Instruments Kontakt basses that I use a lot.
In terms of bass patches, there are plenty to choose from in Dimension Pro, Rapture and Z3TA (in fact it's worth learning Z3TA because you can dial in some great bass sounds in this synth).