Just as a general overview, I'm guessing that the problem is not with the keyboard(as it's only job is to send midi info), but with how you have Cubase configured, and how it's handling the internal sounds. AAR, if you feel it's the controller, rent before buying to confirm. Pretty much any keyboard will suffice to send midi. Doesn't have to be a controller. Also suspect your interface as oposed to the keyboard. I'd suggest finding a Cubase forum and re-posting there. You'll probably have better response.
Best, Paul
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WUDAYAKNOW.. For the first time in my life, I'm wrong again!!