I am simply not quite sure of the logic of buying a lightweight, pretty comprehensive sounding arranger, then lug around a whole bunch of extra gear to expand it.
It would possibly be better to simply get a more expandable arranger (like the PA3X76) with a sampler and extensive voice editing, and you might end up with less extra weight than the module and laptop, less cables, setup hassle, etc., etc..
Overall, most arrangers made nowadays have an outstanding selection of sounds, with many, many variations of each. Not to mention voice editing capabilities and extensive EQ. So far, on the BK-9, I honestly haven't found any sound that truly cries out for another module to get it acceptable.
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An arranger is just a tool. What matters is what you build with it..!