AFAIK the original 808 circuit had 2 oscillators so this one's far from perfect. Now that there are a tonne of more modern drum machines like the Vermona DRM-1 and Machinedrum which have offered similar-but-different sounds, it's still fun to dial in something along the same lines for some variety.
Obviously the 5th overtone is essential here, and I've found that there is a 'sweet spot' with a few notes which really sound great and not far off a downpitched sample of an original 808 CB. Try the low C at +1 and see how you get on. I'd love to sample this note and pitch it up in Ableton or my MPC.
There is a TINY bit of ENV Amt > Pitch required for the first 'dip' but make sure it's really barely there, or you end up with something that really doesn't replicate the original sound faithfully. Brute factor doesn't do much at this low level, but does give a slight soft punch to the whole affair.http://bradyjacobsen.com/microbrute/?id=2616