this is an interesting idea
it does not work now because of the supply:
– the original DIN13-Neutrik cable you mention contains no supply wire
( http://www.matthiasgrob.org/pEE/ppSchema/PaGRcabl.htm )
– I use my Elegy with StringPort with a cable that has a diode in it to make sure that no one sends the 12V from the Polysubbass to some Roland equipment (which probably would not survive)
– it should be possible with a special split cable to feed a GR guitar from the Roland synth and connect the Polysubbass in parallel
the Roland has a lower signal level than the Paradis, so the Polysubbass may cut the end of the bass note
I just talked to Tom about creating a Polybass module that works with the Roland standard. its rather easy but not easy enough to modify the old ones.