I don't know yet how the 990 works, but the 950 allowed to you select a single surround back (in which case you connected the left back) or dual surround backs - a feature that I used for over two years, as I operated a 6.1 setup for a long time.
The ICBM is a somewhat different beast. It was developed when there was only one processing format that supported stereo surround back channels (Logic7, which was then still restricted to Lexicon gear, although it is now also available on some HK equipment). Today there are two such formats, since Pro Logic IIx can do discrete surround back speakers. Dolby EX, DTS ES, and software like the 1050's 6.1 Surround and the 950's Cirrus Extra Surround all generate a single mono surround back signal, which is often split to feed two speakers - for those formats, the ICBM's single surround back input and output loses you nothing.
As for the Model 1050 question, are you experiencing no output from the rear center speaker, or from any speakers? The Model 1050's 6.1 Surround mode only works with Dolby 5.1 signals and the six channel analog input - it can not be applied to stereo signals processed with Dolby Pro Logic - so you should not be getting anything from the rear center speaker when using Pro Logic.