So you currently have a two-channel Audiosource driving the Vandersteen C1's and want to add a second amp? With option one (running the Audiosource amps as bridged mono amps), you would need to make no changes to the bass management. With option two, I would do one of two things: leave the ICBM exactly as it is, or change the center to "large" at the receiver and pass the pre-amp signal through the ICBM.

You are currently running the receiver with the sub off, the mains large, and the center and surrounds small (if I recall the setup correctly), which means that the LFE track and the low frequency data from the center and surrounds is all being redirected to the mains, at which point the ICBM splits the signals between subs and speakers. The system would continue to function exactly as it does now if you were to put an amp in to drive the center. This is an acceptable approach, but I'd probably recommend something slightly different...

Since you have an ICBM already on hand, you can use it to gain greater control over the center channel crossover point. Set the center channel to "large" in the receiver, connect the center channel pre-amp output from the receiver to the ICBM's center input, connect the ICBM's center output to the amp's input, and hook the center to the amp. At this point, you can use the center channel control on the ICBM to select the most appropriate crossover point for the center channel speaker.

This is correct. If a speaker is set to "small" at the receiver, then the receiver applies bass management to the signal using the receiver's crossover point (with low frequency data steered to the subwoofer - or the "large" mains in the event that there is no subwoofer). If a speaker is set to "large" at the receiver, then the receiver does not apply any bass management and the signal bypasses the receiver's crossovers.