I've gone away from simply using only sines as sub. A nice square wave on top of a sine does wonders as it brings out real nice harmonics which you can fine tune with the use of some surgical notch eq'ing... Also, distort/saturate to taste and let those harmonics punch through the mix; you don't want just air bellowing out. Harmonics is the key.
Well, impact is probably the keyword concerning subbass. In terms of hearing and feeling..well.. it's both. On any decent soundsystem you can both hear and feel it. On a club soundsystem it's the thing that sends you reeling but you can absolutely hear it as well.
If you've got a spectroanalyzer try isolating the sound hitting around 30Hz-90Hz. There's you're subbass. Also make sure it's in mono for vinyl cutting purposes.
I dont really know why you would want it detuned, it probably wouldnt cause problems but might sound a bit messy or weird, im not sure...
but if you fuck around with the CRS knob on like osc2 youll notice it just creates like a mid range harmonic i think
as for what sub bass is used for, im no expert but if you had some sort of reece for example,
cut the lows out of it and added a simple sine sub it might create more of an impact than using the bass coming from your reece, lower, and more controlled..
basically the deep shit you feel (and hear) as people have said