I don't know much about the Parts Express components you linked to, I'm afraid, although others around here may have some better info. Paradigm makes a product called the X-30 that may do what you are looking for (I haven't looked at it in a long time, and I'm not sure exactly what sort of I/O it offers), and of course Outlaw's ICBM could do it as well, but in both cases you would need to locate the unit between pre-amp output and amp input for the surrounds, which may not be an option if you are using the Marantz's internal amps to drive those speakers.

Paradigm's X-10 might work best for you, though - speaker level inputs for two channels along with speaker level outputs for both channels and a line level output for a powered subwoofer.