I don’t have any experience with frequency lowering that can go below my MAOF high end frequency. I have tested Resound Enzo Q with Sound Shaper on but it has pretty basic FL with lowest cut-off frequency at 2.5 kH.
I downloaded a Frequency generator app to test the loudness of different frequencies at around 75 dB SPL that corresponds to the upper line of the green shaded area (speech passage at 75 dB SPL). With my old Widexs I can hear sounds that would be of benefit for speech intelligibility up to 1900 Hz for the left and 1500 Hz for the right ear.