An effect to monoize the bass frequencies

Hi, we have two cables, two mains and just one subwoofer with one link connector. We run the sub from the left channel. So anything that’s panned right and have any subs is not produced.

I would like to be able to sum the range bellow certain adjustable frequency so that both the left and right channels are identical in this range.

This is currently possible via using multiple main and matrix channels but I have not found a way to do this on a single channel. The effect Ultra Enhancer does not have adjustable low frequency for this and the set frequency sounds too high.