So you need a second audio interface to get the main out of the maxPatch since all of the 8 output channels are taken by the passive device if I understand correctly ( I have an Apogee Ensemble ).
There are many interfaces you could use with the project. It's really open-ended . So yes it's up to you to come up up with as many outputs as you need. You could make a 6x8 surface and use two outputs for sound if you wanted.
The RME I was using had 8x8 plus a headphone out, I think.
That seems great, Thank you.