I do something very similar.
Sub goes in channel 1 and filters are applied. Output of the BFD goes to a Chase Technologies RLC-1 remote controlled preamp. All signals are mono balanced.
L/R full range signals go to a little Behringer mixer and are mixed to a mono balanced signal. These go into channel 2 input on the BFD, and filters are applied (different filters than the ones used for sub only). That output is also fed to the Chase RLC-1.
I am now able to select (remotely) either input on the Chase RLC-1 and send it to my sub. I am also able to control sub volume
after the BFD, which means I set my BFD's input level and leave it.
Here's some more discussion on my implementation. I really need to complete that thread with some follow up posts, but you'll get the idea. It actually works quite well. The only challenges are 1) getting a Chase RLC-1, and 2) building the custom cables.