808, sub or bass synth
An 808 is a decaying, tuned bass drum that plays the bassline in trap, drill and modern hip hop. A sub bass is a steadier, purer low tone, close to a sine wave, that sits under dubstep, grime and house. A synth bass, from acid squelches to Reese growls, has more harmonics and character in the midrange. Many tracks combine two: a characterful bass for the ear and a clean sub for the body.
Tuning is everything
Bass is where an out-of-tune sample shows most. Find the root note of each sample, which the cards show when the source or an analysis knows it, and map it in a sampler so the keyboard plays in tune. Keep the bass in the key of your track, check it on headphones, and keep everything below about 120 Hz in mono.
Making room for the kick
Kick and bass share the same frequencies. Choose a kick and an 808 tuned to the same note or a fifth apart, shorten the kick's tail, and sidechain the bass so it ducks slightly when the kick hits. On small speakers, a little saturation on the bass adds harmonics that are audible where the fundamental isn't.