Since the gain is a physical setting and preout settings are software, it might be best for setup flexibility to set subs higher and preout +/-0 to start.
Craig probably has a quick and dirty way to set sub gain levels. I measure with full band pink noise and set them 5-8db hotter than the mains. It is probably easiest to set each subs' gain one at a time since they arent matching (treat the co-located subs as one sub and set together) then set the global sub level in bass management. With both front wall sub positions on vs only one there should be a ~3db increase if all pans out at the mlp. You will just have to turn the global setting in the AVR down a bit to your preference.
Its fun if there's no hurry and beer is involved.
