For a simple plain sub, use a subtractor, 1 oscillator, set it to sine, filter fully open.
Adjust ADSR settings to taste.
stick a Scream 4 after it, set it to tape, don't distort it really but make sure its being compressed enough to stay at an even volume (if you want a sharper attack, the compression will accentuate it - just use it in moderation).
Whack a LP filter after it and adjust to taste. Roll any sub off your main bass so it doesn't clash.
Play around and you'll get it to fit.
Also, play with the pitch envelope on the subtractor (depending what you are after)
If you want your sub playing even when the main bass isn't playing then just go ahead and sequence it, if you want it playing at the same time whack it in a combinator along with the main bass (that way it'll trigger at the same time and saves you fannying around with gate CV splitting)
You can also use the sub from your main bass to add character, although I would suggest cutting the lowest freq's out and layering with a clean sub because if your main bass consists of detuned waves, you're gonna get low-end phase issues.
Also, try using other waves as krease suggested. You can even use a saw wave (honest). Play around and practice.
HTH