I have always enjoyed playing manual bass parts, and it bugged me that I couln't twiddle with organ drawbars in real time, because of my left hand being so busy. There are a number of arrangers with digital drawbars that work in real time, but the only ones that have real sliders are the Korg PA1 series and the Roland G70.
The Korg is a hell of a unit, and the ROland is too new to speculate on. There are a handful of them(G70s) on the streets, but too many conflicting reviews to make any opinions on popular concensus.
Soft button drawbars can be found on kn6x-7x, Tyros, PSR2k-3k, 9kpro.... and I THINK the WK series by GEM too.
Since your left hand is busy with the bass parts, maybe a soft button approach is workable ... you can record different drawbar settings in registrations, and swith on the fly with no change to the rhythm parts. It's worth some looking into.
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