When notes drops below the tuning frequency the port can't control the woofer. Does the class D, super high damping factor, mean this isn't as big a problem? I'm hearing bass extension and control that I'd never though possible.
If there is any energy below the port frequency the woofer can still flop around. I seriously doubt that damping factor has anything to do with it. I'm sure if you find a recording with low enough bass you'll still notice it.