Only play subwoofer sounds on Leviathan?

I was wondering if anyone knew of a way to turn down the main Leviathan speaker without also lowering the Subwoofer volume.

My new audio setup has the audio going through a headphone Amp/DAC, then into a pair of headphones and the optical output goes into the Leviathan. I'd like to be able to listen to the headphones and have the feel of Leviathan Subwoofer.