While the BK-7m has a key range parameter for each live part, this feature is missing in the BK-9. This means you can only split lower/upper by using the SPLIT button.
Why is this important (for me)?
Because in the BK-7m I could use 4 live parts flexibly; the "Manual Bass" part could be used like "Lower" if you didn't use the dedicated split function, but instead the key range setting. You could layer a piano plus strings left, and still have two upper parts. Not possible with the BK-9. The key range parameter has been dropped. I'm quite shocked.

The design R&D team for the BK9 must have discussed the elimination of this feature and didn't think enough people needed it in a Kb vs a Module before they created the BK9...........

It's obviously one of those cryptic Roland decisions again, saving some cents on dropping features and being very inconsistent about maintaining and dropping features...
In the Roland forum I found a thread with some Midi / Sysex experiments to find a workaround...

well at least you have a workaround for your needs...but I think Roland needs to re assess what players need in an arranger KB if they are ever going to be back in the game...I hope they do and so does many other players....my Korg M1 & Roland arranger Ra90 module with super card was a killer combination years ago in the early 80's ...comon Roland wake up and jump back in the pool!!

When it comes to playable parts, nothing beats the Roland G70 with 6 parts...

vArranger has 5 for right, and 3 for left

I just found a great workaround myself. The good thing is that there are key ranges in the Midi Rx settings. So I fooled the BK-9 by telling it I was playing from an external keyboard via midi, while in fact playing on the BK-9 itself. Crazy.
Quote from my own post in the other forum:
It WORKS!!!
Plugging the same Midi cable into midi out and midi in is the solution!
Turn off Local in the midi settings, and set the Midi Rx key ranges of MBS and Lower below C4 and for Upper 1/2 from C4 upwards. That's it.
The On/off switches for the 4 live parts work perfectly. That way you have 4 full polyphonic live parts, two left, two right!
If you don't want to connect another instrument and use BK-9 stand alone, that's the solution.

True, but i was talking Hardware arrangers, but Varranger indeed does very well with 8 but misses out on the multipads...(thats where current arrangers spend the other 4 channels)

Most advanced however is the Wersi OAX in advanced mode, where you have 16 channels for each of their 2 or 3 manualls and pdals..


Makes me wonder if Wersi ever made contact with You.. The upcomming V-arranger version with support for all brands of styles could be a great addition to Wersi OAX..

Both of my Ensoniq synthesisers (ESQ 1, SD 1) had key zones available to all available parts. Each part could play internal, external, or both voices. Each part could be enabled / muted on the fly using single and double button clicks. There were 12 parts available on the SD 1 as two banks of 6. Used a highly readable two row display. This is a high level of flexibility with instant access and I really miss this feature.