Let me explain what I did which I doubt will answer all your questions, but maybe provide some insight.
I have the following configuration, which is a bit unorthodox but works out well for me.

I don't have a 'square room' and found excellent coupling while not ideal but 'good' non-localization by using a corner area (adjacent to a main wall) and experimenting with the position of the sub within that area.
Get on the floor with some heavy bass music and crawl around the room to find the point where the bass is difficult to localize. The more areas where it is difficult to localize the better, especially around the main listening area of course.
I have all satellite speakers and a powered sub.
So first of all, I have all satellites set for LARGE so the ICBM does all the work of a crossover. The one exception is the setup in the DVD player, where I left the rear speakers set to SMALL so they would not deal with any frequencies below 100Hz (the default of the player).
The reason I did this is I only use the rear surrounds with the player.
I either listen in 5.1 EXT or Stereo.
I also took the SUB OUT in the DVD player so all LFE information will be directed to the Front MAINS.
Again, the ICBM will crossover the bass info to my sub which is just 'hanging' on the output of the ICBM. No LFE signal input.
A quirk of my system is I have the 3 front channels driven by an external amplifier, which are fed by the 3 pre amps out of my receiver, to the ICBM, and to the amplifier so all frequences above 80Hz are only processed.
Based on this configuration, I have the LCR speakers set to 80Hz, the surrounds set to bypass, (they are still amplified by the receiver but cut off at 100Hz by the DVD player, remember?)
I have no need to use 'recombine' since I am not dealing with any full range speakers.
I do use 'special' for the filter slope because I am using a THX speaker system. The 'special setting' is one you need to experiment with, the same goes for any polarity setting on the sub. The sub is set for external and I use both level settings on the ICBM and sub set at 0 gain which gives me about 80db with a 75 db test on Avia which apparently has a miscalculated subwoofer output signal.
The left/right mono/stereo are mainly for one or two subs. They will have little if any effect on one sub.
I think you might be better to use a quality sub(s) that WILL allow you an external input (ie; from the ICBM).
I would 'round up' the bottom end of your main front left and right speakers, if they go down to 40Hz, I would cut them off at at 60Hz, etc.. and let the sub handle all below that.
This will take some 'load' off your main amps and let the sub work those. This may even make your mains sound more detailed.
The subwoofer level on the ICBM has no effect on the other speakers.
Is your center channel the same size as your front L and R? IF so, they should be 'cutoff' at the point as your L and R front speakers.

[This message has been edited by DaleB (edited November 22, 2002).]

[This message has been edited by DaleB (edited November 22, 2002).]