An ICBM would allow you control over the crossover point used as well as give you bass management in signal paths that would otherwise lack it.

The M&K's in your 5.1 system are probably designed to work with a THX-standard 80Hz crossover, which is probably the crossover applied by the Denon for any digital input. To use an ICBM in that system, you would have to have a DVD player with built-in DD and DTS decoders so you could connect it to the receiver via analog cables. Since you have no analog sources that would otherwise not get bass management and since your speakers are designed to work with the crossover point used by the Denon (assuming the Denon does use an 80Hz crossover when speakers are set to small, which I would hope is true), the ICBM probably isn't a good fit.

The stereo system is somewhat similar - you have a digital connection for CD's that gets its bass management in the 950, and the analog connection (presumably to the left and right channels of the six-channel analog input) gets its bass management from the 950's 80Hz analog crossover. You might consider the ICBM in this system as a way to apply a crossover point different than 80Hz in order to get better results with your MMG's (Outlaw actually has a Magnepan edition of the ICBM with a different set of crossover points that were defined by Magnepan for use with their speakers).