Providing that the front speakers and amps are the same, I would make an audio group with your four front speakers, two stereo pairs, and create a mix-down from the group to send output to the sub. Use the HW cycle notes within octaves within ranks to share the output across your two stereo channels. Assign your main output to this group. Create another group with the two rear speakers, one stereo pair. The sub will receive the full range output but only play sounds within its designed range: the sub plays the low sounds from all of the front ranks.
See
http://www.paramountorganworks.com/downloads/ParamountAudio.pdf from Paramount for instructions on setting this up.
This should make good, probably the best use of your speakers.
Iain