Talking about the stereo sub outputs? They are actually 3) left main + surround + 1/2 center and right main + surround + 1/2 center. Specifically, from the manual (page 14):

Quote:
When the stereo switch is activated, the ICBM gathers all low bass from the right front, right surround, and one-half of the bass from the center channel and center surround channel and places it with the right subwoofer output. Conversely, everything from the left front, left surround, and one-half of the center and center surround are channeled to the left subwoofer output.


The subs themselves are typically located near the mains I believe, although nothing is stopping someone from locating them in the rear. Or for that matter locating four subs in the four corners of the room and using the left sub channel for the front and back left subs and the right sub channel for the front and back right subs...

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