Here is a graph in showing the output with all 3 extension settings.
Red = 14hz setting
Green = 20hz setting
Blue = 28hz setting
Here is as high as I was willing to go @ the 14hz setting without feeling guilty
This test could be felt but definitely not heard. There were objects rattling but other than that I could barely hear the increased output during this test
BTW these are all with sub and mains.