The lowest layer
A sub bass is the part of the low end you feel more than hear. In dubstep, grime, drum and bass and a lot of house, it plays the bassline as a near-pure tone while a mid-range bass or lead carries the character. Because it is so simple, the details matter: a clean start without a click, a steady level and the right key.
Using sub samples
Treat a sub one-shot like an instrument. Load it into a sampler, check its root note (shown on the card where it is known), and play your bassline from the keyboard. Keep it mono, and avoid stacking it with an 808 or a long kick tail in the same register, or the two will fight. Sidechain it lightly to the kick so the kick’s attack comes through.
Watch for phase: two low sounds that start at different points in their cycle can cancel each other out. A sub that starts at zero, with a short fade-in, layers more predictably.