Assuming the specified setup - all small with a sub - you could tell the pre/pro that you have large fronts, then insert the ICBM between the fronts and LFE. Let the ICBM shave the bass from the fronts, redirect to the sub.

Total additional components - 3 cables.

Left, LFE, Right pass through the ICBM. Just be sure the delay is the same for LFE and fronts. As a bonus you can have stereo subs later.

Charlie