Based on some of the reviews I've read, the SW150 does not have a high-pass filter.Does that sub have both a low-pass filter for itself and a high-pass filter for the mains?
He could try using something like these FMOD high pass filters:
https://www.amazon.com/FMOD-Crossover-Pair-High-Pass/dp/B0006N41JW
https://www.amazon.com/FMOD-Crossover-Pair-High-Pass/dp/B0006N41KG
Not sure how well they work.