Hello misterT,
Well, as an exercise to see how this works you can calculate the total impedance of an inductor in series with an (say) 8 ohm resistor. See what size inductor it takes to get a significant increase over the 8 ohms resistive at 30Hz.
Simpler yet, measure the speaker with an Ohm meter. Any ohm meter puts dc through the speaker.
The reason we dont want DC in a speaker under normal conditions is so that during the normal audio we dont want the speaker cone physically biased either in or out or else we can not get the full travel required at high volume. If the speaker normally goes out 1/2 inch and in 1/2 inch from rest position, if we bias it to 1/4 inch out with some DC then that means we only have 1/4 inch left in the 'out' direction. Even though we now have more room to move 'in', it doesnt help because the normal movement is only 1/2 inch 'in' so that means the cone tries to move from 1/4 inch to 3/4 inch and then back to 0 and then inward to -1/4 inch instead of -1/2 inch.
Of course there would also be distortion now that there is very unsymmetrical movement.
You've never biased a speaker with DC to see what would happen? You can also use them as positioning devices for a small apparatus.