I can't tell you exactly how to do this but you can use a passive bass blocker on the wall speakers. We used to use these in car audio, I think it's a capacitor who's size filters out all frequencies below a certain one. This may be risky because you don't know how the sound will be until everything is fired up. You don't want to cross at the wrong frequency.

Audio Control used to make the Phase Coupled Activator that has a 2-way crossover. It uses a card to set the crossover frequency. This piece is also supposed to digitally restore extremely low frequencies.

I'm not familiar with the source you are using, it don't provide any internal crossover?