I'll take a shot at your question -

Set your processor's bass management to all large speakers, with no subwoofer. That way, all bass from all sources will be directed to the mains. With the ICBM, set the rears for 80, and the center to 100. All above these frequencies will go to the mains. The LFE channel will also be directed to the mains, since you have told the processor that you do not have a sub.

Now set the mains to 40, 60, or whatever you choose. All bass from all speakers will go to the sub, along with the entire LFE signal - Remember that you told your processor that you do not have a sub, so the processor has redirected the LFE to the fronts. The ICBM will send the LFE to the sub, combined with the bass from all other channels.

The only drawback that I see is that the LFE control on the ICBM will not control the LFE strength independantly from the overall sub volume, since the LFE is not coming in as a separate signal.

Hope this helps.