I am afraid it can't be done: all Ketron styles that use a sampled groove as second rhytm track cannot slow their tempo below a given treshold because the groove wouldn't sound good anymore. The only solution is to disable the second rhytm track and use the style with just the drum track with midi notes (channel 10): of course the style will sound less authentic, but still acceptable.
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Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.