Hello Stephen
I have sufficient audio channels to dedicate a pair to the sub woofer (which sums he Left and Right channels internally).
I feed the sub audio channels with a mixdown of all the perspectives of the sample set.
I have used a set up where the main perspective output was connected to the sub and the speakers (one pair) were connect to the sub. Circuitry in the sub forwarded the audio output above the cut-off frequency to the main speakers as you are doing.
I prefer having the sub on independent audio channels.
If the speakers you are considering adding are the same as you those In one of your other output perspectives have you considered adding these as another set of speakers to one of your existing perspectives? This will spread more of the sound across the speakers and improve the sound of the organ. It will make more/better use of the new speakers than reserving them for a a few special stops.
Iain