Paolo,
there is only one thing to do: wait until the new Korg hits the street and then try it yourself; meanwhile, you have the chance to try the SD1 plus (or a lot of other keyboards, for that matter): if you liked the X1, it's very likely that you are going to love the SD1.
OK?
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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.