Thread: Choosing a low bass oscillator
For vintage sounding analog goodness I recommend the bubblesound VCOb and/or the uLFO. Really love the bass from those.
Edit: That VCO's little helper module posted above would be cool and reminded me that AM and ring mod-type modulation can create bass-heavy side-bands well. For more grit go with a classic diode/transformer ring mod, for cleaner go with IC/discreet circuits (which are more accurately termed balanced modulators but sound very similar to the classic ring mod).