i have that problem sometimes...what i tend to do is;
Click onto various by the top, drag ssl vapour onto the channel area.Then switch off osc 2, make osc1 a sine wave(smooth wavy line near top)...Then turn the FW knob to zero..after turn the crs knob to zero.
Now u should have a pure bass sine wave....However u may want to assign this to a mixer channel, add a parametric eq and beef it up abit.Also try different presets on a multiband compresser to give it some oomph.
This is FL Only i dont know how to do it with other softwares.
I find the best way for doing subs is loading a sinewave shape into a sampler (=>packs/shapes/sytrus 7 or 8) ....great for getting wub type sub by messing around with the volume/cut/res LFOs. I would then tend to overlay this with the low passed reece mentioned above, gives alot more presence to the bass.