I agree and disagree. DI's can make the top end "tinkly". But I would suggest a decent keyboard amp. Their freq resopnse usually goes 20-20k, and a guitar amp just can't cut it. I also agree that the stereo out is a significant upgrade to most mono outputs. I regularly rent out a MOTIF XS8 and Thraynor K4 amp and the players always make positive remarks regarding the sound. Any effect that are applied internallyto the keyboard comethru loud and clear in the amp. I use the amp as the Keyboardists monitor and have heard many times that they appreciated being able to hear what they are playing in regards to their personal touch.
So you're saying plug the keyboard into an amp like we did in the 60's ? Never would have thought of that. I agree, but not with the need for a special amp. A Fender Twin reverb, Vox AC30, Ampeg portaflex, etc. all work wonderfully for this purpose. The problem is todays thought that everything needs a DI to the board. I look at that as a pretty lame and lazy approach to getting the instrument into the FOH. I sometimes wonder if the art of creating a good mix on stage has been lost by all but the best performers.