Over on the Hsu forum, "Pete_Hsu" says:
"Note that in the 16Hz ULF Trim mode, the subwoofer starts rolling off gently below 16Hz when measured outside and away from reflective boundaries. To take advantage of room gain in small or medium sized rooms, one can adjust the knob to a higher frequency as high as 50Hz. So in many rooms people should get good response down to 10Hz with room gain factored in. In some instances, by utilizing the ULF Trim control to roll off deeper bass, people can get higher output capability over the more common ranges too."