Dennis,
you are right and the difference is even more striking when you consider the price of the X1 with hard disk ($ 2223,05); what is really odd is that the same X1 with hard disk AND vocalist is listed at $ 2086,99 (!!!).
So, either there is a mistake, or there is an odd marketing strategy behind these "crazy" prices (but the price for the SD1 is tempting indeed!)
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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.