The 950 manual indicates a HPF slope of -12db/octave using 2nd order Linkwitz-Riley and a LPF slope of -24db/octave using 4th order Linkwitz-Riley.

The ICBM manual indicates, in ‘normal’ setting, both HPF and LPF slopes are 2nd order Butterworth.

Assuming that the ICBM’s Butterworth arrangement will cause a gentle rise to a +3db boost approaching the crossover point with a sum of the outputs, this would tend to compensate for a loudspeaker characteristic that already is -3db down at the chosen crossover point. If a loudspeaker and sub are down -1db at the crossover point, the acoustic in-phase sum would be +2db. However, a sub and loudspeaker will be slightly out of phase in all but a few ideal spots in the room, so I’m guessing there wouldn’t be the full +2db acoustic boost in reality.

Seems the ICBM designers were making some assumptions about real world equipment/environment and figured if the end result of the crossover would keep things within +/- 1db at the crossover point acoustically, they were doing pretty well – similarly for the 950.