I used this copy of STITI for the comparisons:
http://www.discogs.com/release/12154
This is not DC offset, the drum machine simply exhibits that behaviour. I'm quite sure that a TB909 for example has this. Do you know with what kind of filter one can bring those "spikes" back in line with the rest of the waveform?
It may be that other versions have been mastered slightly differently... I'll check this with my UK copy...