Hi Evangelical,
I'm quite new here too, I recently asked Terry (trtjazz) about MIDI'ing.
I'm not an expert here, but I'll try to explain what I know.
MIDI sends out a signal about what note you're playing. For example, you press middle C, you k-board will export a MIDI message as:
C4
Velocity 95
Modulation 0
Pitch bend +15
....and so on.
Using Midi you can layer your sounds from different KB's or sound modules, saving polyphony. The layer of sounds you can build is only limited by the no. of channels the controlling board can send simutanously. In my XP-80 you can midi out upto 16 channels at the same time.
But the way I did things, I had a PSR8000 and a Roland XP-80, and I successfully linked them together controlling everything wonderfully from the XP-80.
I used only one MIDI cable from the controlling board to the other. I plugged from my XP-80 (MIDI out) then into the MIDI in of the PSR8000.
I then went to MIDI settings in the XP80 to send MIDI messages in channel 1,2 3 and 4. Also I set the MIDI settings in the PSR8000 to receive channels in 1,2,3 and 4.
Then you have to experiment around with the sounds and channels, I normally used the PSR8000's pads and strings in ch3 & 4, while using the piano of the XP80 in ch 1 & PSR8000's EP's in channel 2.
It's pretty complex at first, but it's pretty cool to mix different sounds, and it saves polyphony too!
I think that you can layer voices,in your 9000, since it has 2 MIDI out ports(=32 channels you can control!) and plug them into various sound modules and k-boards.
What I suggest is goto your local music store and buy a midi cable. If you have 2 boards that have good MIDI options/functions, then you're set. MIDI them up together and experiment around. That's how I found out MIDI'ing k-boards!
Well, I hope that helps..
Oh.. and if you're a serious piano player, it'll be a dream if you used a weighted digital piano such as Roland RD-700, Kurzweil PC2 or even a RD-150, since these boards have channel vol. controls built in, so that you can control the relative individual channel volumes from your controller.
Sunny