It sounds like that is just the ICBM doing it's job.
If you set the subwoofer to off on the 811s, the 811s will send any normal subwoofer (LFE) output to the Mains (Front L/R). From there the ICBM is taking anything below your Mains crossover (60) and sending it to the sub. When you hit the sub boost on the 811s, it is boosting it in the mains, which again the ICBM is sending back to the sub.
If this is the case, if you have the sub on off in the 811s, and set the all crossovers to bypass, THEN you should hear nothing in the sub...