Justin, A Milli
's kick sound (according to my ears) is:
a simple, constant-frequency (not changing in pitch) quite-sustained 808, with the high-end rolled-off quite a bit. Get the longest 808 sample you can get, roll-off the highs, and compress it a little bit. Probably 3:1 ratio, -3 or 4 db threshold, 20 ms attack 100 ms release (or close) with Waves Renaissance Comp in Opto/Warm modes. If you want more 'pump', make the release faster (10-20 ms, or so).
I would personally also layer a 909 as sustained-as-possible, mixed in *low*, for a bit more 'punch'.