Well, for what it's worth, the ICBM could remain in the system you described, since all of the modes offered by the 950 that generate surround back data (Dolby EX, DTS ES, and CES) do so in a mono format. You could set the 950 up for 6.1 and let the ICBM split the surround back channel into two for the rear surrounds without losing any information.

As for the popping sound, I'm a bit less useful there. The most likely candidate in my mind for this is the interface between DVD player and 950, and it sounds like the changes you've made are all downstream of the 950. Are you always in surround modes that provide a surround back channel (such as CES) when this happens? If so, can you drop back to straight Dolby Digital or DTS and see if it still does it?