Connect the Subpac using a parallel output on your interface (headphones, aux out, monitor out, etc..) that contains the same signal as the master. Inside the DAW add a second output track that can host an eq plugin that can function as a low pass filter. Send this track to the output that is connected to the Subpac. Adjust the low pass filter to your preferred frequency. Here is a video with an in depth explanation← I want to experiment with the crossover, how do I do this?