Sounds like you’re confusing all the hardware in front of the BFD with the BFD itself. The channel of the BFD you load filters into is the one you have connected to your system.
Generally, no. However, if your response below 30 Hz rolls out gradually, you can "buy" a bit more extension by booting below 30 Hz. But even this depends on your sub. If you have a ported sub, for instance, you shouldn't boost below the port's tuning frequency.EDIT: If my SW only plays between 30-100 hz. and I have my crossover for my mains set to 80hz, do I need any filters below 30hz or above 80hz. for my filter assig. to the 1100?