Couple o'things to be careful about when using the ICBM between your pre-pro and amp:

First, since the ICBM is only 6.1 channels, I would use the EX/ES decoding modes instead of PLIIx processing. Combining two extracted surround-back channels may cause problems whereas dual-mono channels won't. In fact, it would probably be easier to run a mono surround-back from the pre-pro to the ICBM, and then use the Y-splitter at the surround-back output of the ICBM.

Also, I wouldn't use the time alignment feature on the pre-pro. If you combine low frequencies from variously timed signals, it could result in cancellations and re-inforcements at certain frequencies. For example: if you have one speaker that is 4 feet further than another speaker, combining their time-aligned signals will result in cancellation at 140Hz (which has a wavelength of 8 feet and would be 180 degrees out of phase with a signal delayed for 4 feet).