you should average the reading the spl meter gives you for the sub readings; in this case, the steady state reading would be right around 71dB.
the sub frequencies are so low that the spl meter needle is in fact following the driver's movement while it's moving the air in the room.
if the meter's needle could respond fast enough, you'd see the needle swinging wildly 20 times per second if the tone was 20Hz.;
at 1000 Hz, the needle is much too slow to be able to follow the speaker's movement. this is "damping".
