An open baffle sub has a different sound radiation pattern into a room than box subs.
The open baffle H-Frame has a figure of 8 style sound radiation pattern, so very little sound at 90 degrees to the front, back or top. That means less side wall and ceiling interaction with the subs. All box subs either sealed or ported have an Omni radiation pattern so more sound hitting the sidewall and ceiling. If you walk around each type of sub you will hear the difference.
The servo sub controls the driver to make it act more as a piston for better frequency response and lower distortion. Brian has shown a test box with a servo driver and servo amp at RMAF. With the amp off, it is easy to move the cone. With the amp on, no cone movement pushing on it as the servo is actively stopping any movement that is not the input signal.