Bi-Amping: Driving your main L&R speakers in stereo mode only, try them both with and without bi-amping using your present amplifier. Be very careful to listen both ways at "identical" levels as measured at your listening position, not just by volume setting – SPL meter required. Playing back slightly hotter in one of the modes may prejudice your judgment in favor of the 'hotter' mode.

Separating the two sections of the crossover into one section that passes lows and rejects highs while the other section does the opposite and bi-amping (not bi-wiring) would only remove any interaction between the two crossover sections. If ‘harmful’ interaction is minimal, likely you’ll gain little. Unless you have the proper active crossover network to place ahead of the amplification and remove the passive crossovers entirely from the signal path, passive crossover foibles will still be a part of your listening system regardless of whether or not you bi-amp.

If your speakers, with their built-in passive crossovers, and your driving amplifier are all of very good quality, I’m guessing that ‘passive bi-amping’ will not do much for you. But you can try the test above before you buy more amplification. Who knows, it might yield quite a change for the better, or change that is different but not necessarily better, or no seeming change at all.

ICBM: Unless you need it for ‘stereo’ subs (not the same as dual mono subs), the ICBM is likely no longer needed in conjunction with the 990 - with one exception. The ICBM would still come in handy for one or more mono subs if you are running a multi-channel source that has no bass managment in analog mode to the 990 in bypass mode in such a way that the 990's bass management, analog and/or digital, is also bypassed. One might do this to obtain analog bass managment with crossover points selectable at other than 80Hz. (Many believe that stereo subs provide no real benefit, and I think this true most of the time for movies and music where the stereo effect or multi-channel sound placement is created in the mixing board rather than ‘aural imaging’ that is captured ‘live’ by microphone placement. I tried stereo subs, thought I heard some difference on live recordings that were ‘minimalist, purist, stereo’ and ‘minimalist, purist, multi-channel,’ so I’ve kept my stereo sub setup.)