I mainly use the exs24mkII (logic's sampler) and layer bunches of kick samples on top of each other and very the sample length/start/reverse/invert/etc. till i think i got something that works for the track.
I often use a synth set up for a bass sound with a bit of resonance, and on another track a very slight noise hat with quick attack, and on another a bit of mid range bump...then sample them together, add slight distortion/eq/compression/whatever and resample...whatever my fancy at the time.
the sky's the limit.
It can be fun to experiment with kick sounds, but when I sit down to write a loop, i can often get caught up in the details of sound when all I need to do is get caught up in writing the groove.