Thing is, everything sent to each sub out would get the EXACT same reverb, chorus, delay, whatever you put on it. Inside the box, sent to the main outputs, each part can have what you want at whatever level you want...
If you want greater separation, why not simply pan hard right or left? You could have the backing in stereo like usual, pan the RH voices hard right, and LH voices hard left. Voilą!
Subs have always been primarily for separating something out like the bass or guitars, and sending them to their own amp and effects. But chas is right... Virtually NOTHING can have any part sent to any output with it's full insert and send effects on it. That implies a full set of independent insert and most importantly, send effects for every single Part. Most keyboards share the reverb/chorus/delay sends to one 'unit', so the piano gets sent to the same reverb as the horns, for instance. If you want the horns on a separate out with their own reverb, you can't use the built0in, because the piano is in it too!
Using external boxes for effecting the Parts you send to the sub outs is going to involve you in a lot of MIDI programming (so they get the correct effect and wet/dry balance called up for each Performance), or a lot of knob and button pushing, calling them all up by hand...
Me, I'd stick to stereo. Just put your speakers as far apart as possible.
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An arranger is just a tool. What matters is what you build with it..!