Page 108 of the 7705 manual (as it is noted in the PDF itself) covers the issue of how the center and sub signals are derived. There is some sort of illustration on that page but it won't load it properly, so I can't really find the answer, but it does say that the "the center signal is derived from the sum signals of the front channels just in front of the Soft Audio Mute coefficient." The sub signal is the sum of all four channels and is output on the left portion of the I2S secondary, while the center is output on the right portion of the secondary I2S.

Stu