The ICBM already has you covered -- you will have even less to do than you think to use the VTF-2 in stereo mode. Assuming that the mains will be set up for a crossover point in the ICBM (in other words, not set to "bypass"), the low frequency material from those channels will always be redirected to the ICBM's subwoofer output -- even when the GTP 830 is in stereo mode, with its subwoofer output idle.

How I would approach this setup: set all speaker channel delays in the GTP 830 to zero (either 0ms or 0 feet, depending on how they are presented in its menu). Set the 830's bass management to all speakers large, subwoofer on. Switch the sub mode to mono since you only have one sub. Switch the sub lowpass control to normal (unless any HSU sub users around here recommend the steeper 36dB/octave filter). Set L/R recombine to "off" (setting it to on will make the mains receive a full range signal, although if the mains have a crossover point specified there will still be a signal created from those channels for the sub -- resulting in a "double bass" situation that may or may not bother you). Set the left/right channel crossover to 60Hz initially, although you might want to try 40Hz too. Set the center crossover to 80Hz, although you might also try 60Hz and 100Hz if you want. Set the surround crossover to 60Hz. These crossover values are loosely based on a rule of thumb several people around here recommend -- using a crossover point one octave above the -3dB frequency response for the speakers, or double the lower limit and use that -- but in my experience it never hurts to experiment with the crossover point a little. For example, my mains have a frequency response similar to your C-9's, and I have them crossed over at 40Hz in my 950's bass management.

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