Never played the Keytron so I can't comment on it. But I can comment on the 9000Pro WITH the expansion boards - the added improved sounds and polyphony provided by the boards puts it over the top in many areas. Where Andrea placed the 9000Pro second in many of his comments, he might have put it in first place if he'd tried the expansions. To date, I haven't heard a better arranger than the 9000 Pro fully expanded. The acoustic piano exp. board is a must-have - it's richer and fuller than the stock 9000 Pro piano and it gives you another 64 notes of polyphony (using this layered with the stock Live! Strings is a joy). I also like the DX expansion board, which gives you real classic DX sounds like electric pianos and organs plus it adds another 16 notes of polyphony. The analog synth and virtual acoustic boards are my least favorite and they add five notes and one note respectively (you can install any two boards in the 9000 Pro). I would use the analog board if I did classic rock and the virtual acoustic board if I used a wind controller, but since they are easy to swap out they come in handy for studio work.
I also believe the 9000 Pro with the acoustic piano board can flat-out beat the acoustic piano of the Tyros in terms of fullness and total polyphony. With the acoustic piano and DX boards installed, the 9000 Pro has a total polyphony of 208 voices. That's hard to beat. Add the 9000 Pro's sampler and 76-note keyboard and you have a beast that remains one of the dominant arrangers in the world today.
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Jim Eshleman