At 20 Hz, the cone of the ported system will NOT need to travel any more than it did at 40 Hz. Why? The port of the ported system tunes the air in the box to a certain frequency. At frequencies within an octave of that tuning frequency, the air in the box is conditioned to react to just a little excitation by the loudspeaker cone.
Did you like the 2 cubic foot box for it's smoother rolloff, rather that something smaller?